add + line

[bgoodri]

Submission Checklist

• Run unit tests: ./runTests.py src/test/unit
• Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary

Add + line to error message

Intended Effect

Keep people from throwing their laptops into a wall

How to Verify

Parse this

parameters {
#include foobar
}

or do it from rstan, which already contains this fix.

Side Effects

Closes #2507

Documentation

None

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Trustees of Columbia University

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[bob-carpenter]

Thanks. Wouldn't want to be responsible for thrown computers.