Improve debugging for ServiceUnavailable exceptions

I was getting a lot of these in my logs for no apparent reason, and file
uploads were failing:

	{"app":"webdav","message":"Sabre\\DAV\\Exception\\ServiceUnavailable: ","level":4,"time":"2015-01-06T15:33:39+00:00"}

In order to debug it, I had to add unique messages to all the places where
this exception was thrown, to identify which one it was, and that made the
logs much more useful:

	{"app":"webdav","message":"Sabre\\DAV\\Exception\\ServiceUnavailable: Encryption is disabled","level":4,"time":"2015-01-06T15:36:47+00:00"}

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