Parsers silently accepts broken data

[nichtich]

If input is not valid MARC serialization, the result of parsing is not well-defined. I guess from certain point of view this is not a bug but a feature, but I wish to get at least a warning on broken input.

Some examples:

console.log(Marc.parse("abc", "iso2709"))

Record { leader: 'abc', fields: [] }

console.log(Marc.parse("123", "iso2709"))

Record {
  leader: '123',
  fields: [
    [ '123', '12' ],
    [ '123', '12' ],
    [ '123', '12' ],
    [ '123', '12' ],
    [ '123', '12' ],
    [ '123', '12' ],
    [ '123', '12' ],
    [ '123', '12' ],
    [ '123', '12' ]
  ]
}

// broken XML, malformed leader and tag
const marcxml = `<record
<leader>X</leader>
<controlfield tag="1234">YYY</controlfield>
</record>`;
console.log(Marc.parse(marcxml, 'marcxml'))

Record { leader: 'X', fields: [ [ '123', '>YYY' ] ] }