removed confusing 'Error in function' from exceptions

Fixes #640. 


#### Summary:

Removes "Error in function" from exception messages.

#### Intended Effect:

The hope is to confuse users less. Warning messages wrap the exception error messages. The exception messages used to have "Error in function" and now do not.

#### How to Verify:

There are two unit tests that actually test for the error message:
- src/test/unit/math/error_handling/dom_err_test.cpp
- src/test/unit/math/error_handling/dom_err_vec_test.cpp

You can also verify by running a model with an error message.

#### Side Effects:

This will change all error messages.

#### Documentation:

This string is not in Stan documentation (I searched in the latex files). This will need to be updated in CmdStan's documentation.

#### Reviewer Suggestions:

Anyone.