Report error when choosing a folder that isn't a database.

[jcreedcmu]

To Reproduce
Go to the explorer and right-click on a folder that is not a database, and choose CodeQL: Set Current Database. Nothing happens.

Expected behavior
There should be a message communicating to the user that this isn't a database.

Additional context
Making the context menu only appear when the directory is a database in the first place would be desirable, but I don't know if it's possible. The current "when" condition for the command is
"when": "resourceScheme == codeql-zip-archive || explorerResourceIsFolder" and I can't find any refinements of explorerResourceIsFolder.

[jcreedcmu]

Also it would be useful to check if the parent directory of the selected directory is a valid database, just because it's easy to accidentally select e.g. a db-javascript dataset directory instead of the whole database.