Fix fast-fail not logging original error message

[joshpeng-quibi]

resolves #2644

Description

The exception handling in execute_nodes() calls _cancel_connections(). For adapters that aren't cancellable, it goes down a code path that short-circuits the rest of the handling logic via a raise. This PR removes the short-circuit path which allows the handling to reach the print_run_result_error(). Prior behavior never reached there so it never printed the original error messages.

This PR also guards against iterating through a NoneType return from adapter.cancel_open_connections(). In BigQuery's case, it is always None.

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.

[beckjake]

This rocks, thanks for the quick patch! Some minor suggestions and then we'll get this in for 0.17.2.

[joshpeng-quibi]

@beckjake Thank you sir!

[beckjake]

Thanks! Once tests are done, I'll get this merged.

[joshpeng-quibi]

Hm, some weird

createdb.exe : The term 'createdb.exe' is not recognized as the name of a cmdlet, function, script file, or operable program.

in the test.

[beckjake]

I fixed this in the dev/marian-anderson branch but never merged it back here (6e161ab). We'll ignore that test for this PR, I will cherry-pick that commit into the next PR I do for 0.17.2.