When an invalid monitoring channel is provided, do not crash

[locriandev]

Right now if an invalid monitoring channel is configured, or if art-bot does not have access to it (can happen for example when the app is re-installed into the workspace) art-bot crashes. Instead, it should be resilient and log a warning if that happens, but continue as if no monitoring channel was provided

[openshift-ci]

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[mbiarnes]

/lgtm

[mbiarnes]

/retest

[openshift-ci]

@locriandev: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/security	3e2f0a7	link	false	/test security

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