tools/noise/add-profile: Catch missing presets file early

This is required because all commands but the last in a pipe can't make
the script abort if they exit with a non-zero code.
tools/noise/gen-profile: Fix $tethering_enabled logic
tools/noise/subr.sh: Do case-insensitive comparisons homogeneous
tools/noise/subr.sh: Tell user to press Enter after tethering "instructions"
tools/noise/subr.sh: Fix RAW settings query for some Nikon cameras

Reported by:	Fabien Rousseau
tools/noise/subr.sh: Hard-code $shots_seq for now

This variable is not really used for now and I don't know what to use
on Solaris (seq(1) is available on Linux and jot(1) on *BSD and Mac OS
X).
tools/noise/subr.sh: Try to use the proper flag to sed

On Linux: "sed -r"
On others: "gsed -r" or "sed -E"

Reported by:	Moritz Moeller <virtualritz@gmail.com>
gen-profile: Improve messages to notify the user we need RAW images only
tools/noise/subr.sh: Ignore *.jpeg when listing existing images
tools/noise/subr.sh: Add functions to play with darktable's version
tools/noise/gen-profile: Display warning if darktable is < 1.2

We must tell the user that it's new profile is not compatible with
darktable before 1.2.
tools/noise/subr.sh: Check if convert(1) comes from GraphicsMagick
gen-profile: Check channel correctness (esp. $a1 must be >= 0)
gen-profile: Display warning and error messages using colors