chgserver: catch RepoError while loading configuration

Recent share safe work introduced functionality to read share source config file
on dispatch. This can result in RepoError while reading config file as the
shared source might not be present.

`test-share.t#safe` was failing with chg earlier because of this.

Differential Revision: https://phab.mercurial-scm.org/D9462

mercurial/chgserver.py

@@ -504,10 +504,21 b' class chgcmdserver(commandserver.server)'
         the instructions.
         """
         args = self._readlist()
+        errorraised = False
         try:
             self.ui, lui = _loadnewui(self.ui, args, self.cdebug)
+        except error.RepoError as inst:
+            # RepoError can be raised while trying to read shared source
+            # configuration
+            self.ui.error(_(b"abort: %s\n") % stringutil.forcebytestr(inst))
+            if inst.hint:
+                self.ui.error(_(b"(%s)\n") % inst.hint)
+            errorraised = True
         except error.Abort as inst:
             self.ui.error(inst.format())
+            errorraised = True
+
+        if errorraised:
             self.ui.flush()
             self.cresult.write(b'exit 255')
             return