[sbiscigl]

Description of changes:

When CRTHttpClient failed to set up TLS (for example an unreadable caFile, caPath or proxyCaFile), it marked itself bad but nothing checked that flag. It now refuses every request with a non-retryable INVALID_PARAMETER_COMBINATION error when it's bad, sets the flag on proxy TLS failures too, and CreateHttpClient now actually checks the client's operator bool instead of just checking for a null pointer. This is a behavior change for misconfigured clients only: a CRT client with an unreadable CA file now fails all requests.

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• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
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