shout when exiftool is not found

[kapouer]

This could be a warning upon require (not really useful to shout at install time).

[albe-rosado]

Node: 8.9.1, MacOS, just trying the example in the readme, get this error:

events.js:183
throw er; // Unhandled 'error' event
^

Error: spawn exiftool ENOENT
at _errnoException (util.js:1024:11)
at Process.ChildProcess._handle.onexit (internal/child_process.js:190:19)
at onErrorNT (internal/child_process.js:372:16)
at _combinedTickCallback (internal/process/next_tick.js:138:11)
at process._tickCallback (internal/process/next_tick.js:180:9)
at Function.Module.runMain (module.js:678:11)
at startup (bootstrap_node.js:187:16)
at bootstrap_node.js:608:3

[kapouer]

Come to think about it, there are now exiftool-vendored.pl and exiftool-vendored.exe available on npm.
Even better, the dependency on streat could be replaced by exiftool-vendored.js if it handled piping a readable stream to exiftool as streat do.
See also photostructure/exiftool-vendored.js#2