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:

creating symlink '/wfxz2q489c811n08cdqj7ywxm3n4z6m5-nix-eval-jobs-2.24.1.drv.tmp-2971297-324653080' -> '/nix/store/wfxz2q489c811n08cdqj7ywxm3n4z6m5-nix-eval-jobs-2.24.1.drv': Permission denied

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);
+    } catch (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;
     }

src/libutil/file-system.hh

@@ -250,8 +250,6 @@ void setWriteTime(const std::filesystem::path & path, const struct stat & st);
 /**
  * Create a symlink.
  *
- * In the process of being deprecated for
- * `std::filesystem::create_symlink`.
  */
 void createSymlink(const Path & target, const Path & link);
 

src/nix/flake.cc

@@ -936,7 +936,7 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
                         }
                         continue;
                     } else
-                        fs::create_symlink(target, to2);
+                        createSymlink(target, to2);
                 }
                 else
                     throw Error("file '%s' has unsupported type", from2);