Check for zero-length 500 response from the server and produce a suitable error message

In experimenting with PyMISP I am triggering problems on the server I
am using.  Occasionally the server will return a 500 response with a
message indicating an internal error, but more often than not it returns
a 500 response with no contents, and _check_response falls over itself,
generating hard-to-fathom exception from the json internals.

This commit hardens _check_response by detecting zero-length responses
and raising a suitable exception.

Also fix a missing bracket in one of the subsequent exception strings.

pymisp/api.py

@@ -223,13 +223,16 @@ class PyMISP(object):
         """Check if the response from the server is not an unexpected error"""
         errors = []
         if response.status_code >= 500:
-            errors.append(response.json())
-            logger.critical('Something bad happened on the server-side: {}'.format(response.json()))
+            if len(response.content) == 0:
+                raise PyMISPError('Something bad happened on the server-side and there was no content to be decoded')
+            else:
+                errors.append(response.json())
+                logger.critical('Something bad happened on the server-side: {}'.format(response.json()))
         try:
             to_return = response.json()
         except ValueError:
             # It the server didn't return a JSON blob, we've a problem.
-            raise PyMISPError('Unknown error (something is very broken server-side: {}'.format(response.text))
+            raise PyMISPError('Unknown error (something is very broken server-side: {})'.format(response.text))
 
         if isinstance(to_return, (list, str)):
             to_return = {'response': to_return}