Add error handling

[kazuhitoyokoi]

I fixed #32.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

I added appropriate error handling for node generator.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage decreased (-0.5%) to 76.375% when pulling 5183a58 on kazuhitoyokoi:master into 1e7170e on node-red:master.