mercurial/templates/rss/error.tmpl

chgserver: handle ParseError during validate

Currently the validate command in chgserver expects config can be loaded
without issues but the config can be broken and chg will print a stacktrace
instead of the parsing error, if a chg server is already running.

This patch adds a handler for ParseError in validate and a new instruction
"exit" to make the client exit without abortmsg. A test is also added to make
sure it will behave as expected.

{header}
    <title>Error</title>
    <description>Error</description>
    <item>
      <title>Error</title>
      <description>{error|escape}</description>
      <guid>https://mercurial-scm.org/#error</guid>
    </item>
  </channel>
</rss>