TEMPLATE.sh: Die, if BUILD_TMPDIR can't be removed

[donald]

If we don't have the permissions to remove BUILD_TMPDIR (which is quite
likely, when we switch from a test build with a personal user to one
via tools/build.sh with the build user), the current code would ignore
the fact, that the directory can't be removed and continue with a
nonempty directory, possible running into permission problems later.

We could just change

chmod ... && rm -r ...

to

( chmod ... ; rm -r ... )

and the shell would terminate when chmod fails due to permissions.
However, the error message of chmod is different than that of rm:

buczek@afk:~$ chmod u+w /root
chmod: changing permissions of ‘/root’: Operation not permitted
buczek@afk:~$ ls /root/
ls: cannot open directory /root/: Permission denied

I think "Permission denied" is a bit clearer than "Operation not
permitted".

Ignore errors form chmod and try rm in any case, which either
succeeds or aborts the script with an appropriate error message.