stl: improve error messages

[dlecorfec]

Encountered the following error message on a bad STL file:
astisub: building gsi block failed: astisub: atoi of � failed: strconv.Atoi: parsing "\x01": invalid syntax

Decided to replace the second occurence of "astisub" with the name of the target variable, in every place with int or date parsing, so this PR will output:
astisub: building gsi block failed: totalNumberOfDisks: atoi of failed: strconv.Atoi: parsing "\x01": invalid syntax
which makes it easier to pinpoint the bad location in the STL file.

(one could also wonder if it would be worth it to replace the %s of "atoi of %s failed" with a %q, in case of a non-printable string like in this example ...)

Bonus, line 835, removed the "astisub: " prefix which is redundant with the message line 206 in ReadFromSTL().

[asticode]

Adding the field targeted by the error is a good idea, however I'd rather keep the astisub: prefix which is there to indicate which package is submitting the error. Could you update error messages like astisub: totalNumberOfDisks atoi of %q failed: %w and revert the prefix you've removed line 835?

[dlecorfec]

Indeed, before submitting this PR, I checked that the astisub: prefix was already present in the public function (ReadFromSTL) of the package, which calls private functions (parseGSIBlock and newSTLCharacterHandler), where I removed the dups. So there's still an astisub: prefix (on line 199 and 206) indicating the package submitting the error.

[dlecorfec]

I added a change to ignore the Atoi errors when parsing TND and DSN: we encounter some STL with binary "1" instead of ASCII "1", and those values have no interests when convertings subs anyways (plus ttconv also ignores them).