Improve prompt error handling and relative path support #250

Job	Run time
Integration Tests (windows-latest, stdio)	15m 29s
CODEQL_PATH Tests (windows-latest)	1m 52s
Integration Tests (windows-latest, http)	14m 1s
CODEQL_PATH Tests (ubuntu-latest)	54s
Integration Tests (ubuntu-latest, stdio)	11m 21s
Integration Tests (ubuntu-latest, http)	12m 6s
CODEQL_PATH Tests (macos-latest)	1m 33s
	57m 16s

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