Improve error messages for database/instance not found

[haaawk]

• Replace all error messages resulting from database not found to suggest using turso db list to find the ones that do.

• Replace all error message resulting from a specific instance not found to suggest using turso db show existing-db to find the ones that do.

[haaawk]

It seems that most cases of non-existing database are covered:

╰──> ./turso db destroy foo
Database foo, all its replicas, and data will be destroyed.
Are you sure you want to do this? [y/n]: y
Error: database foo not found. List known databases using turso db list
╰──> ./turso db shell foo
Error: database foo not found. List known databases using turso db list
╰──> ./turso db show foo
Error: database foo not found. List known databases using turso db list
╰──> ./turso db replicate foo ams
Error: database foo not found. List known databases using turso db list
╰──> ./turso db change-password foo -p "test"
Error: database foo not found. List known databases using turso db list
╰──> ./turso db show foo --instance-url bar
Error: database foo not found. List known databases using turso db list

The only not covered case I found is:

╰──> ./turso db destroy foo --instance bar
Error: could not find database foo to delete instance from

[haaawk]

For instances both cases I can identify need to be improved:

╰──> ./turso db show t --instance-url foo
Error: instance foo of database t not found
╰──> ./turso db destroy t --instance foo
Error: could not find instance foo of database t