Change error vars (e.g. BAD_SUBJECT_ERR) to strings about nats.js

[aricart]

Hi there thanks for the report. This issue as you have found above, affects both node-nats and node-nats-streaming. The issue should be fairly easy to fix, since we can easily identify all the throw points. Let me do a fix.

Thanks for the report.

[shaneiseminger]

Thanks!

[aricart]

Fix for node-nats-streaming is ed6bdc7ea965eeeaa3b693451c9ca8043c0d2d4b