Display error if metadata directory is missing

[matthias-ronge]

Is your feature request related to a problem? Please describe.
If the metadata directory is missing, the application behaves strangely, but no helpful error message appears (or not immediately).

Describe the solution you'd like
There's a health check in case the index is unavailable. A similar check should be performed to determine whether the metadata directory exists. If not, users should see an error message.

Describe alternatives you've considered
This occurred because the newspaper calendar couldn't be started. At a minimum, the metadata directory should be checked before indexing, before creating a process. The calendar icon shouldn't indicate that the DocType isn't configured for creating processes with the newspaper calendar, but rather that no docType was found, or more precisely, that the METS file is missing.

Additional context
The metadata directory is often located on a NAS, and occasionally the mount gets lost. In such a case, the directory disappears.

[henning-gerhardt]

Should not a removed / missing mount point of a local or network storage throws an alert in the monitoring system itself? That the application should check the existence of a file or directory before accessing it (or at least handle the non access correct) should be the case too but a general monitoring of the whole system should be done first. Checking the existence of a file or directory all the time could be a resource or performance penalty.