Handle "file not found" error in the superclass's errStr().

If a subclass does not override the member function, the superclass's method
takes care of string conversion of "file not found" error. This is a reasonable
default behavior. Subclasses are still able to override to customize error
messages.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@191058 91177308-0d34-0410-b5e6-96231b3b80d8