Fail with descriptive error if package.json is missing

[ryanburnette]

It would be good to fail with a descriptive error if package.json is missing.

Clearly, others have had the same issue.

Loading "Gruntfile.js" tasks...ERROR
>> TypeError: Cannot read property 'dependencies' of null
Warning: Task "rsync" not found. Use --force to continue.

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• #61 Feature/fail with descriptive error#issues60

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[sindresorhus]

Fixed by #61