Return error for an invalid job

[HarshBalyan]

An invalid job is accepted. It fails and then gets retried for the configured number of times.
An invalid job can be defined to have one or more of the following set incorrectly

• queue name
• worker module
• worker function

Fix

A job in the aforementioned case is bound to fail and eventually move to the dead queue.
Therefore, we can return an error before enqueing in such cases by checking the following:

• Check the queue name against the configured pipelines
• Check existence of the worker module
• Check existence of worker function(name + arity)