Very unhelpful error messages for unmapped prefix operators in pratt parsers

[rscarson]

Describe the bug Unmapped prefix operators panic at 'expected operator, found

For example "-1" panics with:

Expected operator, found int_literal

Instead of something like expected operator, found PREFIX_NEG

To Reproduce

• Create a grammar with a prefix operator, for example "-", and a literal
• Do not map this operator with pratt
• Attempt to use the operator; "-1" for example

Expected behavior The error message should report the correct symbol in the grammar, not the next one