statichttprepo: use URLError.reason directly
0b3f4be5c5bf changed str(inst) to inst.reason[0] all the way back
in 2006. URLError.reason is a str and we should have taken that
attribute in its entirety. I think the code was supposed to be
inst.args[1] for compatibility with ancient Python versions.
Python 2.7 always sets .reason, so it should be safe to use
directly.
Differential Revision: https://phab.mercurial-scm.org/D5715