Don't panic on invalid location scheme

Editor assessment

The subject states the guarded failure exactly: an invalid location scheme no longer panics flag parsing. Type fix and the cli/flags scope are accurate, the failing input class is named, and CLI users immediately understand the robustness gain. No body, acceptable since the failing condition and the remedy are both stated. No footers beyond the PR number. Hygiene is clean. A precise, complete message for a contained crash fix.