error message for Lookup

error message for Lookup

Thr proposed change will provide the user with the target Enum Class name as well as up to four possible enum values when a LookupError is raised in the Enum Class.

A user requested that the enum name and possible values are included to the LookupError message to make debugging easier. The criteria included using ellipses for Enums containing more than four values and using ellipses for enum values that were greater than a certain number of characters (for this resolution the limit is 11 characters).

This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [X ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**
Fixes: #4733
Closes: #5490
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5490
Pull-request-sha: 55e76f2ae796b59b7de157cfaae5235dffa359cb

Change-Id: I4541f9efed1c05401587a413e9e748d46938bcd1