Generalize errStr() function so that it can handle any type of errors.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@191069 91177308-0d34-0410-b5e6-96231b3b80d8
Fallback to the default stringize function to show some meaningful error message.

GNU LD driver only understood no_such_file_or_directory error and was showing
just "Unknown Error" for any other type of error. With this patch, the driver
now prints file name and error message string by default.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@191070 91177308-0d34-0410-b5e6-96231b3b80d8