DistUpgradeFetcherKDE crashes on method _warning

[Harald Sitter]

# Impact
DistUpgradeFetcherKDE.py calls self._warning() but in fact _warning is not an instance method of the fetcher but rather a global helper. This causes early unhandled exceptions entirely defuncing the fetcher rendering upgrades with the KDE UI impossible.

This was fixed in https://launchpad.net/ubuntu/+source/ubuntu-release-upgrader/1:15.10.4 and needs to be SRU'd into vivid as it also suffers from the problem.

# Test Case
- run `kdesudo -- do-release-upgrade -m desktop -f DistUpgradeViewKDE`
- should come up with installer

# Regression Potential
None. Fetcher currently crashes 10/10 times

# Other Info
In testing I didn't actually get the fetcher UI itself to display, so I expect there is more wrong. It would however be a good start to not have it crash.

[Brian Murray (brian-murray)]

self._warning is only called if there is a failure to get the release notes and I don't think happens 10/10 times.