Improve "no readable indexLocations found" error message

[jan-niestadt]

This is a broad and confusing error to get.

Should ideally distinguish between these cases:

• no blacklab-server.yaml could be found. Include link to docs, hints.
• blacklab-server.yaml was found, but couldn't be read because of errors.
• blacklab-server.yaml was found, but it contained no indexLocations.
• indexLocations configured but none exist
• indexLocations found but none readable
• valid indexLocations but contains no corpora
• no corpora found, but unreadable directories found under indexLocations.

If running in debug mode (i.e. locally), include the paths to files/directories found.