more readable errors if symlinks cannot be created

Before:

filesystem error: cannot create symlink: Permission denied
[/nix/store/1s2p3a4rs172336hj2l8n20nz74hf71j-nix-eval-jobs-2.24.1.drv]
[/1s2p3a4rs172336hj2l8n20nz74hf71j-nix-eval-jobs-2.24.1.drv.tmp-2772352-1316231068]

Now:

src/libutil/file-system.cc

@@ -602,7 +602,11 @@ std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix)
 
 void createSymlink(const Path & target, const Path & link)
 {
-    fs::create_symlink(target, link);
+    try {
+        fs::create_symlink(target, link);
+    } except (fs::filesystem_error & e) {
+        throw SysError("creating symlink '%1%' -> '%2%'", link, target);
+    }
 }
 
 void replaceSymlink(const fs::path & target, const fs::path & link)
@@ -615,10 +619,16 @@ void replaceSymlink(const fs::path & target, const fs::path & link)
             fs::create_symlink(target, tmp);
         } catch (fs::filesystem_error & e) {
             if (e.code() == std::errc::file_exists) continue;
-            throw;
+            throw SysError("creating symlink '%1%' -> '%2%'", tmp, target);
+        }
+
+        try {
+            fs::rename(tmp, link);
+        } catch (fs::filesystem_error & e) {
+            if (e.code() == std::errc::file_exists) continue;
+            throw SysError("renaming '%1%' to '%2%'", tmp, link);
         }
 
-        fs::rename(tmp, link);
 
         break;
     }