fix(results-store): catch and handle any Exception from exists()

[parkedwards]

resolves #13989

@desertaxle and I were discussing ^, and it looks like we might be running into a problem with remote result_store uses if we expect this exists() check to fire on every task run start. If we don't catch every vendor-specific Exception, it seems like we might prevent those tasks from starting - this is something I observed with the GcsBucket() results store

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[parkedwards]

hey @cicdw - wanted to get your opinion on this. is there a footgun here / am I missing something obvious

[cicdw]

Yup, totally agreed - nice catch and thanks for fixing it!!