updating error message within check_simplex

#### Summary:

Fixes #815. Updates error message for check_simplex. It used to duplicate the name of the variable passed in.

#### Intended Effect:

Just updates the error message. Has the same behavior of throwing exceptions. None of the logic was changed.

#### How to Verify:

Unit tests have been added to check for the format of the message.

#### Side Effects:

None.

#### Documentation:

No need. The exact format of the error messages were never specified.

#### Reviewer Suggestions:

Anyone.