Add an error message if android NDK is not installed

[ArchLinus]

I had trouble figuring out why I couldn't compile for android and realized I didn't have the right NDK version installed. So I figured I would add an error message to make it more obvious.

[dsnopek]

Does this actually lead to the message getting printed in your testing?

In looking into this previously, this code:

def exists(env): return get_android_ndk_root(env) is not None 

... was preventing the generate() from running at all, and so any helpful message there wouldn't get shown (at least this was the case the last time I was looking into this problem).

See #791

[dsnopek]

Yeah, actually, if you look at the top of the generate() function, it already has a helpful error message, the problem is that generate() doesn't get run if the Android NDK isn't setup.

[ArchLinus]

Does this actually lead to the message getting printed in your testing?

Yes, the printing worked, tested on Windows:

$ scons platform=android scons: Reading SConscript files ... Building for architecture arm64 on platform android ERROR: Could not find NDK toolchain at C:/Users/<User>/AppData/Local/Android/Sdk/ndk/23.2.8568313/toolchains/llvm/prebuilt/windows-x86_64. Make sure NDK version 23.2.8568313 is installed. 

whereas before I was just getting linker errors:

... scons: *** [src\variant\vector4i.android.template_debug.arm64.o] The system cannot find the file specified scons: *** [gen\src\variant\string.android.template_debug.arm64.o] The system cannot find the file specified scons: *** [gen\src\variant\string_name.android.template_debug.arm64.o] The system cannot find the file specified scons: building terminated because of errors. 

Yeah, actually, if you look at the top of the generate() function, it already has a helpful error message, the problem is that generate() doesn't get run if the Android NDK isn't setup.

For me it was getting run. The problem is that it doesn't actually check if the files exist on disk, all it does is construct a path to the NDK. I could modify it to do the exists check in get_android_ndk_root if you think that would be a better solution.

[dsnopek]

Ah, ok, so this is handling the case where ANDROID_HOME or ANDROID_NDK_ROOT are set, but NDK can't actually be found at the specified location?

That makes sense! And it seems to be working in my testing :-)

[AThousandShips]

Suggested change

	print("ERROR: Could not find NDK toolchain at " + toolchain + ".")
	print("ERROR: Could not find NDK toolchain at " + toolchain + ".")

Trailing whitespace

[ArchLinus]

Fixed

[dsnopek]

Thanks! And congrats for your first merged Godot contribution 🎉

[dsnopek]

Cherry-picking to 4.2 in #1372

[dsnopek]

Cherry-picking to 4.1 in #1373