Added test that covers error inside async handler.

[Arnovsky]

closes #69

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |      100 |      100 |      100 |      100 |                   |
 index.js |      100 |      100 |      100 |      100 |                   |
----------|----------|----------|----------|----------|-------------------|

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows Code of conduct

[mcollina]

To say that it "closes #69", you'll need to cover the other branch on line 33. Could you add that as well?

Good work so far!

[mcollina]

can you add the --100 option to tap in the package.json? It'll validate that the 100% code coverage is maintained.

[mcollina]

lgtm! Amazing work