Improve the error shown to users on invalid API call responses

[mconf-daileon]

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Author Name: Leonardo Daronco (Leonardo Daronco)
Original Redmine Issue: 1849, http://dev.mconf.org/redmine/issues/1849

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When the gem makes a request to the web conference server that comes with an invalid response (not an XML or an invalid XML), it will raise an exception similar to:

Impossible to convert XML to hash. Error: Missing end tag for 'hr' (got "body") Line: 6 Position: 168 Last 80 unconsumed characters: 

This is ok for the application to know and log, but it's a bad message to show to users.
Users should receive a more generic message similar to: "the web conference server is not responding properly, check the configurations or try again later".