argument_parser: unhelpful message if the given options isn't one of enums

[marehr]

Another problem is that for invalid input values it will already throw before the validator is called.
It will try to look up the invalid string in the enumeration_names and not find it; which results in a throw.
But this is something where we would have to change something in seqan

Originally posted by @eseiler in seqan/iGenVar#78 (comment)

Platform

• SeqAn version:
• Operating system:
• Compiler:

Description

How to repeat the problem

Run this test in the iGenVar project. The problem is the argument 9 for option m, which does not exist.

TEST_F(detect_breakends, test_unknown_argument)
{
    cli_test_result result = execute_app("iGenVar",
                                         "-j", data(default_alignment_long_reads_file_path),
                                         fasta_out_file_path,
                                         "-m 9");
    std::string expected_err
    {
        "[Error] Value parse failed for -m: Argument 9 could not be parsed as type std::string.\n"
    };
    EXPECT_EQ(result.exit_code, 65280);
    EXPECT_EQ(result.out, std::string{});
    EXPECT_EQ(result.err, expected_err);
}

Expected behaviour

The Validator should handle the error message

Actual behaviour

In the iGenVar Project, we found this unhelpful error message:
https://github.com/seqan/iGenVar/pull/91/files#r598704725