Improve prompt error handling and relative path support #263

Job	Run time
CODEQL_PATH Tests (ubuntu-latest)	48s
CODEQL_PATH Tests (macos-latest)	1m 28s
Integration Tests (ubuntu-latest, http)	8m 40s
Integration Tests (windows-latest, stdio)	14m 1s
Integration Tests (windows-latest, http)	11m 30s
Integration Tests (ubuntu-latest, stdio)	9m 8s
CODEQL_PATH Tests (windows-latest)	1m 39s
	47m 14s

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