Add warning if sharp bindings aren't built correctly

[sidharthachatterjee]

Added a friendly error message in case the require to the bindings throws for any reason. The error message looks like this:

"sharp" does not seem to have been built or installed correctly.

- Try to reinstall packages and look for errors during installation
- Consult "sharp" installation page at http://sharp.pixelplumbing.com/en/stable/install/

If neither of the above work, please open an issue in https://github.com/lovell/sharp/issues

Fixes #1624

[coveralls]

Coverage decreased (-0.2%) to 98.282% when pulling 7de714c on sidharthachatterjee:master into 0a3512d on lovell:master.

[sidharthachatterjee]

Tests passed but it seems like the Travis job on Linux (musl) - Node 8 failed with

Segmentation fault (core dumped)
npm ERR! Test failed.  See above for more details.
The command "sudo docker exec sharp sh -c "npm test"" exited with 1.

[lovell]

Thanks Sidhartha, this is a great improvement.

[lovell]

I've added some more specific help in commit 687795c based on common error messages. Further additions/improvements are always welcome.

(I've also added commit 90a0382 to hopefully prevent/reduce the occurrence of the unrelated test failure you experienced.)