Improve the message when the share expiration date update fails

Description

If "expire share" fails, then the caller of the testing API currently gets a message: Share expire failed

which is not very helpful.

Also send the message that goes with the exception that happened in the API response.

I am working on a PR that changes some sharing behavior, and noticed that it will be nice to have this improvement.

Now I get a response body like:

<?xml version="1.0"?>
<ocs>
 <meta>
  <status>failure</status>
  <statuscode>400</statuscode>
  <message>Share expire failed: Cannot set the requested share permissions for textfile0.txt</message>
  </meta>
  <data/>
</ocs>

which gives a better clue about what might be going wrong.

Checklist:

[x] Latest changes are published according to instructions in README.md

owncloud / testing

Improve the message when the share expiration date update fails #200

Description

Checklist: