Revert "Print a better error when we encounter unset variables"

This reverts commit 0c68d801d1e517c3ae07895aaff449bec5b4e889.

Reverted due to unintended side-effects.
Fail with a better error when doing out-of-bounds READ_2DA_ENTRY_FORMERs.
tolower, bugfix and optimisation

- Don't try to open files as directories
- Don't tolower a file and then process it again
Fix tolower's dependencies

- Remove the dependency on Util
- Add batteries included
Print a warning when BASH_FOR bails out early on account of an exception