Split FieldError::ModulusOverflow variant

[divergentdave]

The FieldError::ModulusOverflow variant is currently used in three situations:

1. try_from_bytes(): This is the original use of this error, and it directly matches the existing error message. It decodes an integer from a byte slice, compares the integer against the modulus, and returns a field element.
2. valid_integer_try_from(): This converts from one integer type to another, and additionally compares against the field modulus.
3. Bit vector encoding/decoding helpers: These compare the requested bit vector length against the bit length of the modulus.

Using ModulusOverflow in the bit vector case is a misnomer, as we're checking a different condition. Additionally, in valid_integer_try_from(), no byte slice is involved. This PR introduces a dedicated error variant for the bit vector helper, and rewrites the existing message for ModulusOverflow, so that error messages better align with their causes.