ctest_build: Do not crash on bad generator name

If creation of the global generator fails, return early with an error
message instead of trying to use the generator and crashing.

Add a CTestTestBadGenerator test to cover this case.

Reported-by: Mathieu Malaterre <malat@debian.org>
Bug-Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=747306
Merge topic 'ctest-bad-generator' into next

54111286 ctest_build: Do not crash on bad generator name
Fix desktop file

Drops the icon file extension. Fixes the desktop file validation message:

"CMakeSetup32.png" for key "Icon" in group "Desktop Entry" is an icon
name with an extension, but there should be no extension as described in
the Icon Theme Specification if the value is not an absolute path
Merge topic 'desktop-icon' into next

39c94af9 Fix desktop file