Capture and display error

[unional]

Currently, it only displays the resulting error, which is not that useful:

jest-watch-exec: executes on-start: yarn build
{ Error: Command failed: yarn build
error Command failed with exit code 1.

    at ChildProcess.exithandler (child_process.js:288:12)
    at ChildProcess.emit (events.js:182:13)
    at maybeClose (internal/child_process.js:962:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:249:5) killed: false, code: 1, signal: null, cmd: 'yarn build' 

Should capture stdout/stderr and display them.