Summary:
Switch all code in the logging library from using `ERROR` to `ERR`,
and remove the `ERROR` LogLevel entirely, even if it is not already
defined as a macro.

Previously the code kept `ERROR` available as a LogLevel name if it had
not already been defined as a macro (which is done by common Windows header
files).  However, this made for inconsistent behavior, and made it easy to
write code that would not be portable to Windows.

This diff fully drops the `ERROR` name for consistency across platforms.

Reviewed By: wez

Differential Revision: D5288600

fbshipit-source-id: 8d2d52e955959c278345fc9c2086c7cacf9660f9