ATH_CHECK: Do not print FATAL message for RECOVERABLE

This MR consists of several parts, which could have been done separately, but since each of them has the risk of requiring updates to various reference files, it's better to have them all merged in one go:

1. Extend the CHECK unit test to include a custom StatusCode (Timeout). Include the category name in the AthStatusCode::message().
2. CHECK macro: Instead of printing the StatusCode numeric value (e.g. 0) print the message (e.g. FAILURE). This is especially useful for custom StatusCode categories.
3. The CHECK macros would print an ERROR message in case the StatusCode under check is RECOVERABLE, otherwise a FATAL message. Change this to always print an ERROR message. FATAL messages should be left to the framework when deciding to abort the job. This point was discussed (and agreed) in today's reco meeting (https://indico.cern.ch/event/808941/). Our https://twiki.cern.ch/twiki/bin/view/AtlasComputing/ReportingErrors policy will be updated accordingly.
4. Added a unit test for all macros to ensure the "checking" logic is correct.

cc @leggett @ssnyder @mhodgkin @rbielski @goetz