Fixed _error_checker in rest client

Restclient's method '_error_checker' for response codes 500 and 501 gets key
error exception when 'resp_body' is a string and passes conditions for
containing of keys.
Also added fix to rarely raised Exceptions after separation main and base
exceptions for two modules.

Closes-Bug: #1280767
Closes-Bug: #1289190
Partially-implements: bp unit-tests
Change-Id: I4b909d008940f8a96efc922daac64a086819c78a
Allow failing logs with errors on a per-log-file basis

Change-Id: I382a173b9e2ad4cf2d6c5761db62722cb8828e55
Remove unused build_url function in data_utils

This commit removes build_url function in data_utils because there is no
user of this code. The using code of this function was removed at
 I1102859c871b8f16c284bdf7fba0f4346d50a63e .

Change-Id: I8989ac12f36f342ca1066cc65f7a427a83de721f
Merge "Fixed _error_checker in rest client"
Merge "Allow failing logs with errors on a per-log-file basis"
Merge "Remove unused build_url function in data_utils"