stdio: add machinery to identify failed stdout/stderr writes

Mercurial currently fails to notice failures to write to stdout or
stderr. A correctly functioning command line tool should detect
this and exit with an error code.

To achieve this, we need a little extra plumbing, which we start
adding here.

mercurial/error.py

@@ -122,6 +122,12 class CapabilityError(RepoError):
 class RequirementError(RepoError):
     """Exception raised if .hg/requires has an unknown entry."""
 
+class StdioError(IOError):
+    """Raised if I/O to stdout or stderr fails"""
+
+    def __init__(self, err):
+        IOError.__init__(self, err.errno, err.strerror)
+
 class UnsupportedMergeRecords(Abort):
     def __init__(self, recordtypes):
         from .i18n import _