Suppress the bigquery job sql in exception messages (#2383)

[beckjake]

resolves #2383

Description

The bigquery library appends the job sql to the exception message, but we don't want that.

There are a couple other paths this could go down instead, I'm not sure which is the most future-proof:

• look at exception.args[0], which will have the original message it got
• look at exception.errors, which will be a list of dicts. Each member of the list (apparently) will have a message key that is the original message.

The other options depend upon google not changing internal workings of their libraries, this option depends on google not changing their error messages. Hard to say what's better, but I kind of assume the output is more reliable than internals!

I figure if the "string contains" check fails it will fail in an annoying-but-not-critical way, as opposed to AttributeErrors in exception handlers.

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[drewbanin]

LGTM!