Closes #461

Force templates to be consumed completely,
propagate error appropriately

...makes better error messages :-)

Notice the breaking change in 'applyElem', where $if(...)$ conditions
in templates now will throw errors if their field 'fail'ed (instead of just
being 'empty')!

...when a more important error prevails.

Also not throwing from 'if' conditions any more,
only logging those errors to the debug screen

* boolFields used outside of 'if'-conditions now get a "stack trace"
  using a new 'NoField' they don't have to rely on 'error' any more
* templates applied to their own file get proper description
  (did use incompatible paths/identifiers before)
* renamed 'compilerFail' to more descriptive name

Closes #507 (actually was fixed by 9ec43a6 already, this just adds the test)

See #462 (comment)
and below for detailed explanation

Also abstracted out `testCompilerError` in the test suite,
and added a `compilerTry` that is much easier to use (and specifically, to
branch on) than `compilerCatch`