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...this doesn't actually test the conversion result, might be worth reflecting
upon later.

This rationalise the way we report errors from libsbml in a few different
places.

...to make the work with custom datatypes easier.

Also:

* replace `@warn` with `@error`
* actually test the logging message in `readSBML`, instead of showing an extra
  message saying to ignore it.

(also ensures compatibility with the macro)

This potentially saves a lot of headache with various custom functions or stuff
where support may be complicated/ambiguous. In this precise case, we're
allowing it because the user needs to `@register` the factorial function with
Symbolics manually, so that it can be translated properly.

how did it even pass before?

Closes #54

(Uses a few new models from EBI repository.)

this checks whether the fix of #16 works

well, who'd say it's hashing the filename?