Lukasa authored May 26, 2020 and Nathan Harris committed May 27, 2020

Motivation:

parseInteger did not distinguish between not having enough bytes for an
integer and not being able to parse the integer that was present. This
was a bit tricky for code internally, where some call sites had extra
code looking for spooky action at a distance in order to determine if
the integer failed to parse.

This is unnecessary: parseInteger is sufficiently aware of what's going
on to address this problem itself.

Modifications:

- Added a new parser error (acceptable as we haven't tagged 1.0 yet).
- Throw it from parseInteger if the integer is invalid.

Result:

parseInteger clearly communicates if the integer failed to parse.