Generate better error messages for string.format.

library/string.format.m:
    We used to parse format strings, and pair up format specifiers with
    the list of input poly_types, using semidet code, throwing
    exceptions with generic messages if the semidet code failed.
    This diff switches to using det code that returns a list of
    situation-specific error messages. Since errors after the first
    may be caused by the first error, we now throw exceptions whose
    message is a detailed description of the first error, but that
    decision is easily changeable.

    Simplify some existing code.

tests/general/string_format_test_{2,3}.{exp,exp2,exp3,...}:
tests/invalid/string_format_bad.err_exp:
tests/invalid/string_format_unknown.err_exp:
    Update the expected error messages.