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.
Add braces to suppress Clang's dangling-else warning.

These violations were introduced in r191049
DebugInfo: GDBIndexEntry*String conversion functions now return const char* for easy llvm::formating

This was previously invoking UB by passing a user-defined type to
format. Thanks to Jordan Rose for pointing this out.