Bug 1261397

Description of problem:
When I executed 'rhpkg switch-branch' inside a dist-git repository, the command fails with message that is too vague:

$ rhpkg switch-branch rhel-6.8 
Could not execute switch_branch: Could not check out rhel-6.8

One is not able to figure out what is the problem. I had to use directly 'git checkout' to get some meaningful error message. 

Version-Release number of selected component (if applicable):
rhpkg-1.26-1.fc22.noarch

How reproducible:
always if you have the the proper situation in your repository

Steps to Reproduce:
1. branch $(A) contains tracked file $(file)
2. branch $(B) does NOT contain file $(file)
3. switch to branch $(B)
4. touch $(file)
5. switch back to branch $(a) using 'rhpkg switch-branch'

Actual results:
$ rhpkg switch-branch rhel-6.8 
Could not execute switch_branch: Could not check out rhel-6.8

Expected results:
some meaningful error message helping me to resolve the issue.

Additional info:
Git gives such a meaningful error message.

$ git checkout rhel-6.8
error: The following untracked working tree files would be overwritten by checkout:
	bind98-CVE-2015-5722.patch
Please move or remove them before you can switch branches.
Aborting