Fix #1015: vzdump: send email on early errors

Fix #1015: vzdump: send email on early errors

VZDump->new() dies when a tmpdir or dumpdir is configured
but does not exist. At this point the error is not being
reported via email.

This also moves the instantiation of VZDump into the worker
since new() can now call sendmail() on error.

Additionally rather than only showing a single error if both
tmpdir and dumpdir don't exist, both are included in the
message.