better error handling which always reports the right file name

src/runtime/c/pgf/pgf.h

@@ -55,6 +55,7 @@ typedef struct {
  * - If the exception was caused by external factors such as an error
  * from a system call, then type will be PGF_EXN_SYSTEM_ERROR and
  * the field code will contain the value of errno from the C runtime.
+ * The field msg will be NULL or it may contain a file name.
  *
  * - If the exception was caused by factors related to the GF runtime
  * itself, then the error type is PGF_EXN_PGF_ERROR, and the field