JS-Evaluator : Lack of descriptive error message while pushing to store

While developing functional tests for refactored js-evaluator, we noticed that a push for a modified library sometimes fails with the following error:

'Compilation succeeded but failed pushing library to store: Cause: 

'Write failed for location: {math}. Cause:'

'mkvWrite: exceeded timeout of 10s when trying to write {Name:math} . Reason:

 Error writing to remote store'

"Error writing to remote store" can happen due to 5 different reasons two of which are a call to metakv.Set().

We should be propagating the lowest level of error messages so that the complete error path is clear (especially with the logging disabled in current evaluator version).