add catch block to worker message handler, resolves #6

[danielbank]

This PR adds a catch block to the worker's message event handler. If the call() receives this message, it rejects its promise and downstream code can detect it.

[danielbank]

I didn't add any unit tests but I didn't see any in place. If you're open to it, I might take a stab at that

[bminixhofer]

Hi, thanks for this PR! LGTM, I'll merge this later today.

Regarding Unit tests: That would definitely be useful!

There are integration tests using Cypress in quality/tests/test.js which compare the output of tractjs with that of tract.

I am not sure how to go about adding unit tests. Would you use Cypress too? Or some node script? (in that case, tractjs would have to be made compatible with Node.js first ;) )

[danielbank]

I was thinking something like Mocha (https://mochajs.org/). Some other alternatives are Jest (https://jestjs.io/) or Jasmine(https://jasmine.github.io/) but Jest is more centered around UI and Jasmine is quite a bit older at this point. They run via Node but I don't think it is a problem to have them as a dev dependency. I'll try and take a look sometime this week

[bminixhofer]

Sounds good, the problem is that at the moment tractjs uses fetch internally so it doesn't work in Node out of the box. This used to be a bigger problem (when it was only Rust code) but now that there is a Typescript wrapper anyway it's actually really easy to fix.