tests/test-revlog-raw.py.out

error: ensure ProgrammingError message is always a str

Since this error is internal-only and a runtime error, let's give it a
treatment that makes it behave identically when repr()d on both Python
2 and Python 3.

Differential Revision: https://phab.mercurial-scm.org/D4545

local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed