Handle errors in expansion of response files

Previously an error raised during an expansion of response files (including
configuration files) was ignored and only the fact of its presence was
reported to the user with generic error messages. This made it difficult to
analyze problems. For example, if a configuration file tried to read an
inexistent file, the error message said that 'configuration file cannot
be found', which is wrong and misleading.

This change enhances handling errors in the expansion so that users
could get more informative error messages.

Differential Revision: https://reviews.llvm.org/D136090