* Fix `DBInstanceNotFound` error message
Changed from `Database` to `DBInstance`, which is actually what comes back from AWS.
* Remove duplicate test
The removed test actually fails if run in isolation because `rds2` is not a
valid boto3 client service. The reason this test never caused CI to fail is
because it is redefined later in the test suite, effectively making it dead
code that will never run.
Duplicate test has been removed and the remaining test has been improved
with more explicit asserts.
ae2865d559