Look for setup.php, and error message improvements.

+ The check-environment script checks for the COLLECTIVEACCESS_HOME
  variable and ensures it is set to a directory; now it additionally
  looks for the setup.php file under $COLLECTIVEACCESS_HOME.
+ Improved consistency and value of error messages.
+ Split exit codes so that the actual error condition can be
  determined by calling script (currently not used anywhere, but
  makes sense in a general way).