Create TMPDIR if it doesn't exist.

The first time I ran "./bin/civiprj create", drush failed because TMPDIR
didn't exist.
Ignore autogenerated snapshot directory.

We don't want to store snapshots in the repository because they are
auto-generated.
Fixed problem where errors were being ignored.

It seems like eval $(cmd) doesn't exit if cmd fails and we want to exit if
the command fails, so we'll store the results of $(cmd) in a variable and
that line will exit if cmd fails, and then we'll eval on the next line.
Removed some repeated code.

PRJDIR/build gets used in several places, so we'll just call it BLDDIR.
Fixed crash because DRUPAL_ROOT doesn't exist.

I'll bet this used to be DRUPAL_ROOT and got changed to WEB_ROOT at some time
and these lines were missed.
Merge pull request #6 from civicrm/master-cherrypick-2

Small fixes to get working on Ubuntu