Update connections.py with retry logic for 502

[patkearns10]

resolves #562

Description

Checklist

• I have read the contributing guide and understand what's expected of me
• 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 opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

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Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the dbt-bigquery contributing guide.

[dbeatty10]

Thanks for opening this PR @patkearns10 !

You found exactly the right piece of code to update to add BadGateway 502 to the list of exceptions to retry.

Could you update the description to say resolves #562 instead of Potential Fix for #562? It might feel presumptive, but it will accomplish two things:

1. ensure that the issue and the PR are linked on each page respectively
2. will close the issue automatically if the PR gets merged