More descriptive tooltip when building tunnels in "half-slopes"

[Roboron]

Motivation: Recently a user asked on the Steam forums what "suitable ground" and "straight slope" mean when building a tunnel, because the user was already trying to build a tunnel on a "straight slope" but was confused because it still was no "suitable ground".

==> https://steamcommunity.com/app/434520/discussions/0/4263206669450879709/?tscn=1629127619

Of course the problem was that the slope is half-height, and thus it didn't have the necessary height for a tunnel to be built. I guess this a usual newbie error. But it kept me wondering if the game can't do better about telling the user in such a case. This patch replaces the "No suitable ground!" tooltip with "The slope is not high enough!" just in case the terrain is only "half suitable"

Text is merely a suggestion, replace to your liking.

[Leartin]

The slope might be too high in double height paksets, so the BASE text should be "The slope doesn't have the right height". Paksets may use "The slope is not high enough" or "The slope is too high" as their translations, IIRC only one of them can happen in any given pakset.

[Roboron]

Well thought Leartin, thank you. I replaced it by your suggestion. 

Paksets may use "The slope is not high enough" or "The slope is too high" as their translations

Interesting. I didn't know paksets could overwrite base translations!

[prissi]

In (with slight modifications and different text) in r10016.