Provide more information when string merge failed

[nojaf]

Currently, when you format a file that contains multiple defines, it might happen that fails due to

Fantomas is trying to format the input multiple times due to the detect of multiple defines.
There is a problem with merging all the code back together. Please raise an issue at https://github.com/fsprojects/fantomas/issues.

This tells you that what the problem is but doesn't pinpoint anything useful at all.
In essence, one (of more) compiler defines combination will contain fewer hash directives (#if FOO) than the other results.
And so, we assume it cannot be merged.

It would be beneficial if we at least told which combination had how many fragments inside the merge function.

fantomas/src/Fantomas/Utils.fs

Lines 62 to 71 in 2fcb03a

	let merge (newline: string) (a: string) (b: string) : string =
	let aChunks = splitWhenHash newline a
	let bChunks = splitWhenHash newline b
	if List.length aChunks <> List.length bChunks then
	Dbg.print (aChunks, bChunks)
	failwithf
	"""Fantomas is trying to format the input multiple times due to the detect of multiple defines.
	There is a problem with merging all the code back together. Please raise an issue at https://github.com/fsprojects/fantomas/issues."""

If you know what combination returns fewer pieces then you can try and trace down where some trivia was not assigned or printed.