Better error descriptions if CSV is invalid

[kspurgin]

Improve error message for invalid character encoding in CSV
0e1b7e4

• As long as CSV is getting invalid_encoding error(s), only the new
  message will be shown. This is because encoding errors sometimes cause
  structural errors that cascade through the file, causing hundreds of
  error messages about structure in which the message about encoding
  gets lost. Often if the encoding is fixed, the structural errors
  disappear. This makes it obvious/clear that fixing the invalid
  character encoding is the required.
• Shows the first occurrence of each invalid character in context, so
  users can more easily find/fix the issues

Show errors for Csvlint-valid files CSV library can't parse
6b3586f

Resolves #213
Currently, you are allowed to proceed to run the pre-processing job
with such a file, but the job fails when processing hits a row that
causes a CSV::MalformedCSVError. Unfortunately, this often occurs at
the very end of the file.

This commit adds a separate csv_parse_validator_for(batch) method
that is only called if Csvlint reports the file is valid.

CSV-Parsing the whole file before creating the batch may seem like
overkill, but (a) I can't come up with a better way to catch all
possible issues; and (b) it seems better than using system
resources/user time to pre-process a whole batch that is only going to
fail at the end.

Thinking forward: If we are eventually moving to using the database to
store information about each row and its state/status, then this step
can be reworked to serve a dual purpose: (1) prepare each row to be
added to database if we are able to prepare all rows successfully;
and (2) if not able to prepare all rows successfully, show error
messages and destroy batch.

[kspurgin]

Splitting this what this was originally going to cover into two PRs