Improve error message on unsupported command

[gagern]

This is a spin-off from #526 (comment).

If input contains some unsupported command nested in some group or environment, the parser will complain about the fact that the group resp. environment isn't ended with one of the expected symbols. The most likely scenario, however, is that the user didn't really mean to end the environment, but did expect the function in question to exist. So it would be better to report that appropriately.

Source locations: if this check finds no matching function, null will get returned which propagates up the tree and out of the parseExpression invocation where it triggers the observed error message. The !throwOnError handling might be the right place to generate a more useful error message, too. Doing so may make behavior with or without throwOnError more consistent as well.