Ignore invalid download paths instead of panicking

[FrogAi]

Summary

• Treat a download path with no submenu separator as an ignorable input instead of panicking.

Motivation

getDataForPath panics when a download path contains no . separator. Every caller reaches it from Download, which Settings.Handle runs as a goroutine, so an unrecognized MapdInputType_download value does not fail the download — it takes down the process. mapd runs that download alongside the 50 ms loop publishing speed-limit and curve-speed output, so a single malformed command string from a fork ends live driving output until the daemon is restarted.

The path string is fork-supplied (docs/inputs.md), and the failure needs nothing exotic: "ohio" instead of "north-america.ohio" is enough.

A well-formed path naming a region that does not exist already behaves harmlessly — the menu lookup misses and returns a zero LocationData, so the download resolves to zero files. This change gives the malformed case that same outcome and logs why, rather than treating it as unrecoverable.

Behavior

Download path	Base 68813e05	Head
north-america.ohio	Downloads the region	Unchanged
ohio (no separator)	Panic; daemon exits	Warning logged; resolves to zero files
""	Panic; daemon exits	Warning logged; resolves to zero files
north-america.not-a-place	Zero files, no message	Unchanged

Validation

• An audit-only harness confirmed the panic on Base for "ohio", "", and "." — both directly and through countTotalFiles, the call path Download takes before any download begins — and confirmed Head returns a zero value with a warning on all three while a populated menu entry still resolves to its real record.
• go build ./..., go vet ./..., and go test ./... pass on Head.

Compatibility

• No schema, settings, CLI, published-message, or offline-map format changes.
• No signature changes; getDataForPath is unexported and its callers are untouched.
• Valid and unknown-but-well-formed paths behave exactly as before.
• Does not conflict with #107: that PR restructures this file but leaves getDataForPath byte-identical, so the two apply cleanly in either merge order.
• The panic is the only crash addressed here. It does not add validation feedback for unknown region names, which remain a silent zero-file download on both Base and Head.