Fix 'bad object 0*40' error (fixes #376)

If the oldrev is all zeroes, do not supply it as an argument. This
will default to everything from the beginning (which is what we want
in that case)
Catch exception during initenv (fixes #368)

When no MTA is installed, or no /usr/sbin/sendmail executable exists,
submin does not know how to send an e-mail. Instead of bailing out of
the installation we catch the Exception and print a warning and
continue as much as we can.

The user can recover by requesting a password reset for 'admin'.

For the record, whenever this happens, the following commands have
to be executed to finish the installation:
 * submin2-admin <env> upgrade hooks no-fix-unixperms
 * submin2-admin <env> unixperms fix
 * submin2-admin <env> apacheconf create all
 * submin2-admin <env> trac init # (optional)
Add install instructions (refs #368)

Explain what users can do in case their e-mail has not been received.