chgserver: catch Abort while parsing early args to shut down cleanly

_loadnewui() calls dispatcher functions, which may raise Abort if unparsable
arguments are passed in. The server should catch such errors and translate
them to the "exit 255" instruction so the client can finish the IPC session
cleanly.

Spotted while porting the chg client to Rust.

mercurial/chgserver.py

@@ -425,6 +425,13 b' class chgcmdserver(commandserver.server)'
             self.ui.flush()
             self.cresult.write('exit 255')
             return
+        except error.Abort as inst:
+            self.ui.error(_("abort: %s\n") % inst)
+            if inst.hint:
+                self.ui.error(_("(%s)\n") % inst.hint)
+            self.ui.flush()
+            self.cresult.write('exit 255')
+            return
         newhash = hashstate.fromui(lui, self.hashstate.mtimepaths)
         insts = []
         if newhash.mtimehash != self.hashstate.mtimehash: