replace remaining uses of panic_impossible with error in oct-parse.yy * oct-parse.yy (unexpected_token): New function. Use it to replace panic_impossible in functions that use switch statements to handle a subset of operator tokens. (end_token_as_string): Eliminate default case in switch over enum values. Handle previously missing token::arguments_end case in switch. (base_parser::parse_fcn_file): Call error instead of panic_impossible.

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