Description of problem:
I upgraded one machine from f19 to f21 using [1] and substituted dnf for the yum. Then running:

# dnf install system-release-workstation
Error: package generic-release-workstation-21-7.noarch requires generic-release = 21-7, but none of the providers can be installed

Yum output is much more helpful:
# yum install system-release-workstation
...
Error: fedora-release-workstation conflicts with fedora-relase-server-21.0.16.noarch
Error: fedora-release-server conflicts with fedora-release-worsktation-21-0.16.noarch

By still not being optimal the yum output lead me easily to conclusion that I have to do:
yum swap system-release-server system-release-workstation

which worked correctly.

[1] https://fedoraproject.org/wiki/Upgrading_Fedora_using_yum

Version-Release number of selected component (if applicable):
dnf-0.6.2-1.fc21.noarch

How reproducible:
Always

Steps to Reproduce:
1. Upgrade f19 to f21 by using dnf (there is very probably more simple reproducer)
2. dnf install system-release-workstation

Actual results:
Cryptic error message

Expected results:
Something that will guide user that she/he needs to swap system-release-server to system-release-workstation, at least something similar to the yum output.