frontend: re-raise errors to API instead of passing null

get_build_record and get_srpm_build_record methods returns
None in case of exception or empty task which results in
null response in API endpoints. Backend worker
don't expect null value, tries to unpack it and fails.
Re-raising the error will let know backend that something
went off

frontend/coprs_frontend/coprs/views/backend_ns/backend_general.py

@@ -190,7 +190,7 @@ def get_build_record(task, for_backend=False):
 
     except Exception as err:
         app.logger.exception(err)
-        return None
+        raise err
 
     return build_record
 
@@ -237,7 +237,7 @@ def get_srpm_build_record(task, for_backend=False):
 
     except Exception as err:
         app.logger.exception(err)
-        return None
+        raise err
 
     return build_record