convert: differentiate between IOError and OSError on commitctx()
The IOError exception is overloaded to mean 'this file was deleted in
the current commit'. Separate the code that handles IOError and file
deletion from general OSError exceptions. The latter are real errors,
but IOError is not always a throwable error.
This solves the accidental marking of files as 'deleted' in commits that
try to write for example in .hg/store/data revlogs that the current user
has no permission to modify (a normal OSError that should abort the
current commit).
Changed by pmezard: use getattr() to be on the safe side.