Don't log from inside the logger

This deadlocks ProgressBar, e.g.

  # nix run --impure --no-substitute --store '/tmp/nix2?store=/foo' --expr 'derivation { builder = /nix/store/zi90rxslsm4mlr46l2xws1rm94g7pk8p-busybox-1.31.1-x86_64-unknown-linux-musl/bin/busybox; }'

leads to

  Thread 1 (Thread 0x7ffff6126e80 (LWP 12250)):
  #0  0x00007ffff7215d62 in __lll_lock_wait () from /nix/store/9df65igwjmf2wbw0gbrrgair6piqjgmi-glibc-2.31/lib/libpthread.so.0
  #1  0x00007ffff720e721 in pthread_mutex_lock () from /nix/store/9df65igwjmf2wbw0gbrrgair6piqjgmi-glibc-2.31/lib/libpthread.so.0
  #2  0x00007ffff7ad17fa in __gthread_mutex_lock (__mutex=0x6c5448) at /nix/store/h31cy7jm6g7cfqbhc5pm4rf9c53i3qfb-gcc-9.3.0/include/c++/9.3.0/x86_64-unknown-linux-gnu/bits/gthr-default.h:749
  #3  std::mutex::lock (this=0x6c5448) at /nix/store/h31cy7jm6g7cfqbhc5pm4rf9c53i3qfb-gcc-9.3.0/include/c++/9.3.0/bits/std_mutex.h:100
  #4  std::unique_lock<std::mutex>::lock (this=0x7fffffff09a8, this=0x7fffffff09a8) at /nix/store/h31cy7jm6g7cfqbhc5pm4rf9c53i3qfb-gcc-9.3.0/include/c++/9.3.0/bits/unique_lock.h:141
  #5  std::unique_lock<std::mutex>::unique_lock (__m=..., this=0x7fffffff09a8) at /nix/store/h31cy7jm6g7cfqbhc5pm4rf9c53i3qfb-gcc-9.3.0/include/c++/9.3.0/bits/unique_lock.h:71
  #6  nix::Sync<nix::ProgressBar::State, std::mutex>::Lock::Lock (s=0x6c5448, this=0x7fffffff09a0) at src/libutil/sync.hh:45
  #7  nix::Sync<nix::ProgressBar::State, std::mutex>::lock (this=0x6c5448) at src/libutil/sync.hh:85
  #8  nix::ProgressBar::logEI (this=0x6c5440, ei=...) at src/libmain/progress-bar.cc:131
  #9  0x00007ffff7608cfd in nix::Logger::logEI (ei=..., lvl=nix::lvlError, this=0x6c5440) at src/libutil/logging.hh:88
  #10 nix::getCodeLines (errPos=...) at src/libutil/error.cc:66
  #11 0x00007ffff76073f2 in nix::showErrorInfo (out=..., einfo=..., showTrace=<optimized out>) at /nix/store/h31cy7jm6g7cfqbhc5pm4rf9c53i3qfb-gcc-9.3.0/include/c++/9.3.0/optional:897
  #12 0x00007ffff7ad19e7 in nix::ProgressBar::logEI (this=0x6c5440, ei=...) at src/libmain/progress-bar.cc:134
  #13 0x00007ffff7ab9d10 in nix::Logger::logEI (ei=..., lvl=nix::lvlError, this=0x6c5440) at src/libutil/logging.hh:88
  #14 nix::handleExceptions(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::function<void ()>) (programName="/home/eelco/Dev/nix/outputs/out/bin/nix", fun=...) at src/libmain/shared.cc:328
  #15 0x000000000046226b in main (argc=<optimized out>, argv=<optimized out>) at /nix/store/h31cy7jm6g7cfqbhc5pm4rf9c53i3qfb-gcc-9.3.0/include/c++/9.3.0/ext/new_allocator.h:80

src/libutil/error.cc

@@ -62,35 +62,28 @@ std::optional<LinesOfCode> getCodeLines(const ErrPos & errPos)
         LinesOfCode loc;
         try {
             AutoCloseFD fd = open(errPos.file.c_str(), O_RDONLY | O_CLOEXEC);
-            if (!fd) {
-                logError(SysError("opening file '%1%'", errPos.file).info());
-                return std::nullopt;
-            }
-            else
+            if (!fd) return {};
+
+            // count the newlines.
+            int count = 0;
+            string line;
+            int pl = errPos.line - 1;
+            do
             {
-                // count the newlines.
-                int count = 0;
-                string line;
-                int pl = errPos.line - 1;
-                do
-                {
-                    line = readLine(fd.get());
-                    ++count;
-                    if (count < pl)
-                    {
-                        ;
-                    }
-                    else if (count == pl) {
-                        loc.prevLineOfCode = line;
-                    } else if (count == pl + 1) {
-                        loc.errLineOfCode = line;
-                    } else if (count == pl + 2) {
-                        loc.nextLineOfCode = line;
-                        break;
-                    }
-                } while (true);
-                return loc;
-            }
+                line = readLine(fd.get());
+                ++count;
+                if (count < pl)
+                    ;
+                else if (count == pl)
+                    loc.prevLineOfCode = line;
+                else if (count == pl + 1)
+                    loc.errLineOfCode = line;
+                else if (count == pl + 2) {
+                    loc.nextLineOfCode = line;
+                    break;
+                }
+            } while (true);
+            return loc;
         }
         catch (EndOfFile & eof) {
             if (loc.errLineOfCode.has_value())
@@ -99,7 +92,6 @@ std::optional<LinesOfCode> getCodeLines(const ErrPos & errPos)
                 return std::nullopt;
         }
         catch (std::exception & e) {
-            printError("error reading nix file: %s\n%s", errPos.file, e.what());
             return std::nullopt;
         }
     } else {