[Error] Add FileError helper; upgrade StringError behavior

FileError is meant to encapsulate both an Error and a file name/path. It should be used in cases where an Error occurs deep down the call chain, and we want to return it to the caller along with the file name.

StringError was updated to display the error messages in different ways. These can be:

1. display the error_code message, and convert to the same error_code (ECError behavior)
2. display an arbitrary string, and convert to a provided error_code (current StringError behavior)
3. display both an error_code message and a string, in this order; and convert to the same error_code

These behaviors can be triggered depending on the constructor. The goal is to use StringError as a base class, when a library needs to provide a explicit Error type.

Differential Revision: https://reviews.llvm.org/D50807

llvm-svn: 341064