mu4e: improve mail-retrieval error handling

Add two new customization variables:
  mu4e-index-update-error-continue
  mu4e-index-update-error-warning

With these, we can configure what happens when the mail-retrieval
program finishes with a non-zero exit code.

Make the default to warn but continue; it seems quite some users got
bitten by the old behavior of not updating after an error (which may
only be a pseudo-error). offlineimap/mbsync do not document their exit
codes very well, unlike fetchmail.

Also update manual for this.