(maint) Use root cause for generic 500 errors.

Improve the utility of the catchall error middleware by having it use
the exception's root cause, if possible.
(maint) fix indentation in acceptance test
(NC-192) Improve group name uniqueness error msg.

When the client tries to create a group that violates a database
uniqueness constraint, return them a 409 Conflict response that explains
which fields of the group violated the uniqueness constraint, the values
of those fields, and the name of the violated constraint.

This is implemented by introducing a new exception-catching middleware,
`wrap-uniqueness-violation-explanations`, that catches a slingshot
exception thrown by `create-group*` whenever the group creation
transaction throws either:
  - a PSQLException with the postgres uniqueness_violation error code;
  - a BatchUpdateException whose root cause (the last exception in the
    chain) is the above.
(NC-192) Add more checks for uniqueness violations.

Factor out the uniqueness error handling into something reusable, and
use it to improve error messages on any transaction where we might run
into a uniqueness violation. Since keeping the offending entity around
all the places we want to use this is a bit of a hassle, remove that
from the exception handling and the error response returned to the
client.

None of the changes to the transactions in postgres.clj are functional,
they're just indentation changes due to the wrapping.
Merge pull request #58 from aperiodic/nc-192-uniqueness-violation

(NC-192) Uniqueness Violation