Use PyErr_Format instead of sprintf

[akx]

Just noticed a couple places using sprintf() instead of PyErr_Format, figured those could use unifying.

[radarhere]

While I don't see a problem, it's worth being aware that this is rewriting the fix to GHSA-7534-mm45-c74v

[akx]

@radarhere Thanks! Good note. I looked into it: PyErr_Format calls _PyErr_FormatV, which calls PyUnicode_FromFormatV, which uses PyUnicodeWriter (an automatically growing buffer) with unicode_from_format, so buffer overflows shouldn't be possible with PyErr_Format.

I reverted the (inadvertent, my bad!) template string changes back to %.10s though, so no chance of MemoryError either.