Improve error messaging when multiple processes connect to db concurrently.

[nicktobey]

Today I tried to run some CLI commands on a database while I had a debug process suspended in my IDE.

The error message read:

error on line 1 for query DROP TABLE t: cannot update manifest: database is read only
cannot update manifest: database is read only

It's good that we detect and disallow concurrent modifications to the database, but our messaging could be better. Is it possible for the CLI to distinguish between a db that's locked because of concurrent access and a db that's actually read-only?

[timsehn]

@macneale4 will fix this with his current work as you'll be able to connect with the CLI to a running database.

Gonna close it in favor of the uber issue #3922