Correctly create error on binary child process error

[pgcalixto]

Currently, when the child process spawn is closed (child.on('close', ...)), the rejected error is created with custom properties that are not native to Javascript's Error.

This code is at https://github.com/fagbokforlaget/pdftohtmljs/blob/master/lib/pdftohtml.js#L97:

reject(new Error({code: code, msg:`${self.options.bin} returned an error.`, params: self.options.additional}));

Because of this, when pdf2htmlEX returns an error, pdftohtmljs only shows the following message without explicit details about the error:

Error: [object Object]
    at ChildProcess.child.on (/usr/src/app/node_modules/pdftohtmljs/lib/pdftohtml.js:97:18)
    at ChildProcess.emit (events.js:198:13)
    at maybeClose (internal/child_process.js:982:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:259:5)

The message above should display explicit details about the error.

[iapain]

@pgcalixto Thanks for PR 👍. Released in pdftohtmljs@0.5.3.