Function builds fail silently

[erquhart]

- Do you want to request a feature or report a bug?
sort of both, I'd call it a bug

- What is the current behavior?
When running Netlify Dev, Netlify Lambda is watching and rebuilding my functions on the fly. If I break a function build, the only way to tell is that the console doesn't log the "changes detected, rebuilding" line from Lambda.

If I don't notice this, everything will still work because the last successful function build will continue to be used until I delete built files. This means I can spend a fair amount of time refactoring a function, not realizing it broke a while ago.

- If the current behavior is a bug, please provide the steps to reproduce.
Using Netlify Lambda, write a broken function that will produce a failed build while Netlify Dev is running.

- What is the expected behavior?
The full error message and stack trace from Netlify Lambda should be allowed to print to console.

- Local Environment Information
Using cli 2.20.2

  System:
    OS: macOS 10.15
    CPU: (8) x64 Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz
  Binaries:
    Node: 12.13.0 - ~/.nvm/versions/node/v12.13.0/bin/node
    Yarn: 1.19.1 - /usr/local/bin/yarn
    npm: 6.12.0 - ~/.nvm/versions/node/v12.13.0/bin/npm
  Browsers:
    Chrome: 78.0.3904.87
    Safari: 13.0.2

[erezrokah]

This was fixed in #988 and requires latest netlify-lambda due to netlify/netlify-lambda#239