ssh: fix flakey ssh errors on BSD systems
There's been a persistent issue with flakiness on BSD systems (like OSX) where
the 'no suitable response from remote hg' message would sometimes not appear.
This was caused by one of the earlier calls failing with a "IOError: Broken
pipe". Catching those errors and printing the same message removes the
flakiness.
Differential Revision: https://phab.mercurial-scm.org/D687