Problem: Vim becomes unusable after opening new window in BufWritePre
event.
Solution: Call not_exiting(). (Martin Tournoij, closes#2205)
Also for "2q" when a help window is open. Add a test.
Problem: When making backups is disabled and conversion with iconv fails
the written file is truncated. (Luo Chen)
Solution: First try converting the file and write the file only when it did
not fail. (partly by Christian Brabandt)
Problem: writefile() continues after detecting an error.
Solution: Bail out as soon as an error is detected. (suggestions by Nikolai
Pavlov, closes#1476)
