GB229 20.20.03 Allow std::format to throw for argument / format string mismatch LWG 3340

[wg21bot]

Formatting functions don't allow throwing on incorrect arguments
std::format is only allowed to throw if fmt is not a format string, but the intention is it also throws for errors during formatting, e.g. there are fewer arguments than required by the format string.

Proposed change:
Allow exceptions even when the format string is valid. Possibly state the Effects: more precisely.

[tituswinters]

LEWG in Belfast: Victor Z to file an LWG issue, suggesting that LEWG approves of the ability for std::format (et al) to throw when given format strings that are invalid for that invocation (rather than only throwing on inputs that are invalid format strings).

[vitaut]

LWG issue: https://cplusplus.github.io/LWG/issue3340

[JeffGarland]

LWG has not prioritized 3340 -- will consider in Prague

[mclow]

LWG3340 accepted on Thursday AM in Prague and LWG3336 will be marked as "Resolved by 3340"