Build errors should not break watch script

[danfinlay]

Currently, a build error will interrupt the watch cycle.

Reproduction steps:

• mkdir test && cd test && mm-plugin init to setup
• mm-plugin watch to use watch script
• Edit index.js to have some invalid JS.
• Observe watch script breaks.

Desired behavior:

Watch script should report errors, but be ready to retry on file change still.

[rekmarks]

Fixed in 0.3.4