BigQuery: Hide error traceback in BigQuery cell magic

[plamut]

Closes #6848.

This PR makes sure that long tracebacks are hidden on exceptions when using %%bigquery cell magic, and only the relevant info is printed out.

How to test

• Run IPython from the virtualenv and load the %%bigquery cell magic:

  %load_ext google.cloud.bigquery
  

• Run a query that triggers an error, e.g. a syntax error, unknown dataset, etc. Example:

  %%bigquery 
  SELECT 1 SELECT
  

Actual result (before the fix):
An error is printed out, including the full traceback of internal calls.

Expected result (after the fix):
Only the relevant error info is printed out, i.e. without the traceback, as the latter can be considered an internal detail of the cell magic.

Things to discuss

• It might not be desirable for the exception to be swallowed and replaced with a simple output to stdout. Should not make any difference for users, unless some of them use %%bigquery cell magic programmatically?
  No, as no one should be using the %%bigquery magic outside of a Jupyter environment

[tswast]

Thanks. IMO, no one should be using the %%bigquery magic outside of a jupyter environment, so swallowing the exception is OK.