mediawiki.page.watch.ajax: Fail early if updateWatchLink is called wrong

mediawiki.page.watch.ajax: Fail early if updateWatchLink is called wrong

If the exposed function is called from a gadget or user script but
no watch/unwatch link was found the function fails even if the
first parameter is a valid jQuery object as required by the
documentation. $link.attr(...) returns null and null.match(...)
fails.

A very simple example why this can happen is as follows:
mw.page.watch.updateWatchLink( $( '#ca-watch a' ), 'watch', 'loading' );
mw.page.watch.updateWatchLink( $( '#ca-watch a.loading' ), 'unwatch' );
(starts the spinning loading animation and tries to stop it
afterwards but may fail if something went wrong, e.g. the user
clicked the star).

The action parameter needs to be checked because it is used to
build an ID and a message key. Bad values turn the page in an
unrecoverable state (the watch link gets an empty action=, the
label turns into something <undefined> and no script can recover
that broken state since the ID turned into whatever). While such a
check is not necesarry in most cases it is here, because the
function is exposed.

Change-Id: I6ee9a7ee6b7c0fc7a5444674afd1ed6f8cacc858