Better error handling for failed connections

[alexjreich]

If a connection for an adapter isn't successful, and exception should be raised and the application should exit. Right now, missing connections mean the models aren't loaded, causing weird downstream errors. This is confusing for people who don't see the earlier log warnings about connection errors.

[loppear]

since _connectDb can also fail for incorrect model definitions (which also leads to no models being loaded), this will helpfully catch another category of problems too.