Enhance errors to help operator fix problems in CSV

[reliztrent]

See also #11

Make more human-friendly error messages for situations where the operator needs to fix the CSV:

• A required field is missing
• A controlled vocabulary field is using an disallowed value
• An disallowed character is in the upload CSV
• A file is encoded incorrectly or is the wrong file extension

[jayanthdeejay]

We have done some work on the batch loader for error logging, download files if URIs are provided and a swat of other such features if you'd like to checkout here.