Especially if one tries to interact with all marked messages there could be
the case that not all headers are fetched yet, hence the messageInfo is still nil.
This segfaults a lot of commands which in principle only need the uid to complete.
If we switch to uids, this issue can be alleviated for those commands.
* :save takes a path and saves the current message part to that location
* :pipe is the same as pipe on the account page, but uses the current
message part rather than the whole email (ie :pipe gzip -d)
* Refactored account:pipe and extracted common pipe code to
commands.util.QuickTerm
* Added helper command aerc.PushError
