More detailed error message for missing files

More detailed error message for missing files

Make it possible to report the actual OS error from File::exists(),
and use this to provide more detailed error messages when a file
provided on the command line does not exist.

Make some unexpected I/O errors fatal instead of silently returning
false from the existence check (the caller can still decide to just get
false even in error conditions, which is used in some older code).

Provide context information for more exceptions that can be thrown
during command-line parsing (in particular, for the file I/O exceptions
that this code can generate, but also some other cases are improved).

Change-Id: Ie39858f4722221a7d98c41aad950c9ee7fde4bbd