Fix #3619 and correctly handle exceptions in XMLRPC

[splitbrain]

This ensures that any exception happening during XMLRPC processing is
signalled correctly to the client as XML encoded error message.

[Klap-in]

@satwareAG-ironMike and @rozhuk-im could you review/test this?

[rozhuk-im]

2022-02-04 22:26:37	/usr/local/www/wiki/inc/Remote/XmlRpcServer.php(24)	IXR\Exception\ServerException: XML-RPC server not enabled.
  #0 /usr/local/www/wiki/lib/exe/xmlrpc.php(13): dokuwiki\Remote\XmlRpcServer->__construct()
  #1 {main}

in log, but dokuwiki android client got

XmlRpc call wiki.getPageHTML error: org.apache.xmlrpc.client.XmlRpcClientException: Failed to parse server's response: Expected methodResponse element, got html

I expect that client will receive: "XML-RPC server not enabled." error message and do not get error messages in log.

[splitbrain]

@rozhuk-im did you actually use the correct branch?

[rozhuk-im]

4ef5d38 + 4b86732

[splitbrain]

Ah you're right. I aborted too early... should work now.

[rozhuk-im]

Now client receive: XmlRpc call dokuwiki.getTitle error: org.apache.xmlrpc.XmlRpcException: XML-RPC server not enabled.
and I see no errors in log.
Looks like fixed, thanks!