Use consistent terminology in our error messages

**Issue by [ozgune](https://github.com/ozgune)**
_Friday Jan 29, 2016 at 17:34 GMT_
_Originally opened as https://github.com/citusdata/citusdb-private/issues/108_

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We currently use inconsistent terminology across our warning and error messages. I recently observed this as part of #93.

A few examples are:
- We use "partitioned" and "distributed" interchangeably. Our documentation uses "distributed".
- We use "partition column" when our documentation refers to it as "distribution column".
- We use "cannot plan" and "cannot perform distributed planning" to both refer to distributed planning.
- We use "sharded modification" or "distributed modification" to refer to the same concept.
- We use "master" or "coordinator" node to refer to the node that keeps distributed metadata.

We also have a few other inconsistencies in other places. One error message says "unsupported error type 'a' ". However, we no longer have an argument 'a' for our UDFs.

No files, tests, or entry points are named. Start by inventorying warning and error messages alongside the documentation terminology; completion means identifying and updating the inconsistent terms, including the obsolete UDF argument reference, with coverage for the affected messages.

Written by the indexing model from the issue text.