Add format_provider for the Error class

Summary:
The formatter supports the same options as the string-like classes, i.e. the
ability to truncate the displayed string. I don't anticipate it would be much
used, but it seems consistent.

Reviewers: zturner, clayborg

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D28519

llvm-svn: 291759