Provide better diagnostics when mkfifo fails by including the file path it was called on.

Diagnostics could be further improved by pre-testing that the file path
exists and is readable and writable before attempting to use it.
Fixes #13: Detailed error of why mkfifo failed will now be visible on parent's STDERR.

Note: When the mkfifo call fails, the parent process crashes. I'm not
sure if that's the intent.
Fix #14: Provide better diagnostics when call to spawn phantom fails.

We now provide the full system call details, to help under file and path issues.
Improve variable name 'filename' to be fifoFile.

'filename' is extremely generic.
The updated name 'fifoFile' communicates clearly that the file in
question is the the FIFO created to communicate with phantom.
Add a comment to clarify that file is run by Phantom

In particular, it's useful to note that it has it's own 'fs' module,
whose API differs from that of the core node.js module.
Document the internal API of using "!" to indicate a rendering failure.
Improve the name of the 'st' argument.

Before it wasn't clear what 'st' argument was for.

I updated it to be requestStatus, which makes it a little easier to
understand what the code is doing.
Add a couple comments to explain these code paragraphs.
Merge branch 'better-diagnostic-for-mkfifo'
Merge branch 'explain-the-bang'

Conflicts:
	phantom-process.js