Add parameter validation to verify incoming jobs prior to execution

[ccmoloney]

We should be doing at least some validation of the parameters being passed to the Execute operation in order to sanity-check incoming requests prior to submitting the job to AWS batch for processing.

These checks could include checking to ensure that the incoming WPS execute request does not include outputs and features that we are not supporting.

Validation should provide some meaningful error messages when common parameter/request problems are detected.

[pmbohm]

#215