Throw error if item content is invalid

[mutantcornholio]

I've wasted some time trying to understand root cause of random EBADF's in my tests.
Like this:

Error: EBADF, bad file descriptor
    at Binding.<anonymous> (/Users/cornholio/dev/veendor/node_modules/mock-fs/lib/binding.js:463:13)
    at maybeCallback (/Users/cornholio/dev/veendor/node_modules/mock-fs/lib/binding.js:30:18)
    at Binding.read (/Users/cornholio/dev/veendor/node_modules/mock-fs/lib/binding.js:455:10)
    at ReadFileContext.read (fs.js:342:11)
    at FSReqWrap.readFileAfterStat [as oncomplete] (fs.js:387:13)
    at /Users/cornholio/dev/veendor/node_modules/mock-fs/lib/binding.js:75:16
    at /Users/cornholio/dev/veendor/node_modules/mock-fs/lib/binding.js:38:9
    at _combinedTickCallback (internal/process/next_tick.js:67:7)
    at process._tickCallback (internal/process/next_tick.js:98:9)

And was surprised finding that I was reading file, which contents were set to undefined in config.

After this PR, I get error on calling mockfs:

Error: Unsupported type: undefined of item package.json
    at populate (node_modules/mock-fs/lib/filesystem.js:148:15)
    at Function.FileSystem.create (node_modules/mock-fs/lib/filesystem.js:192:9)
    at mock (node_modules/mock-fs/lib/index.js:59:27)
    at Context.beforeEach (test/unit/install.test.js:41:9)

[mutantcornholio]

@tschaub what do you think of this?

[mutantcornholio]

@tschaub ping?

[mutantcornholio]

@tschaub ?

[tschaub]

This looks good @mutantcornholio. I'll make an update to get the tests passing and then pull this in.