Improve prompt error handling and relative path support #260

Job	Run time
Integration Tests (ubuntu-latest, http)	9m 0s
CODEQL_PATH Tests (macos-latest)	1m 34s
Integration Tests (ubuntu-latest, stdio)	8m 53s
CODEQL_PATH Tests (ubuntu-latest)	57s
Integration Tests (windows-latest, http)	13m 10s
CODEQL_PATH Tests (windows-latest)	1m 36s
Integration Tests (windows-latest, stdio)	13m 41s
	48m 51s

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