'ue4 setroot' saves the root even when specified directory is NOT valid

[sleeptightAnsiC]

$ ue4 setroot jndksfjnaskldjfnksda
Set engine root path override: /home/kornel/Projects/ue4cli/jndksfjnaskldjfnksda
Using user-specified engine root: /home/kornel/Projects/ue4cli/jndksfjnaskldjfnksda
Warning: the specified directory does not appear to contain a valid version of the Unreal Engine.
# return code 0

$ ue4 root
Using user-specified engine root: /home/kornel/Projects/ue4cli/jndksfjnaskldjfnksda
/home/kornel/Projects/ue4cli/jndksfjnaskldjfnksda
# return code 0

This should not happen. Setting root override to a non-valid Engine directory should fail.
Currently it always saves the override no matter what, and only prints warning.

In #65 I already changed warning to error by throwing UnrealManagerException.
Now I need to also prevent the override from being saved.

This is a bit tricky to fix as currently we save the override first and then validate it afterwards:

ue4cli/ue4cli/UnrealManagerBase.py

Lines 39 to 54 in fed71c1

	def setEngineRootOverride(self, rootDir):
	"""
	Sets a user-specified directory as the root engine directory, overriding any auto-detection
	"""
	# Set the new root directory
	rootDir = os.path.abspath(rootDir)
	ConfigurationManager.setConfigKey('rootDirOverride', rootDir)
	print('Set engine root path override: {}'.format(rootDir))
	# Check that the specified directory is valid and warn the user if it is not
	try:
	self.getEngineVersion()
	except:
	print('Warning: the specified directory does not appear to contain a valid version of the Unreal Engine.')