Various alert-related fixes
gnutls_alert_send_appropriate: do not send alert to peer on all errors
alert: associate unsupported curve alerts with handshake failure
Checklist
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Code modified for feature
Reviewer's checklist:
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Any issues marked for closing are addressed -
There is a test suite reasonably covering new functionality or modifications -
Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
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This feature/change has adequate documentation added -
No obvious mistakes in the code