Better error reporting

[zzril]

Error messages are pretty generic atm. Especially the (quite common) case where the notes' lengths in a bar don't sum up to 1 currently leads to a cryptic "Internal interpreter error".

Improve these error messages and point the user to what exactly was wrong.

Note: In order to determine if the (cumulative) length of the notes in a bar is correct, we depend on the floating-point arithmetic to be exact for powers of 2. We should verify that is indeed correct on all destination platforms, and otherwise implement a fallback solution (e.g. a tolerance window of a couple of samples to still accept a bar).
Keep in mind that the notes' lengths will get more complicated once #36 is implemented.

[zzril]

Part of this is currently addressed in #42 . More specifically, error return codes are introduced in that PR to describe what exactly the problem was.
However, actually printing out sensible error messages based on the returned error codes is not implemented in that PR.