Improve error for missing Python

[danmoseley]

Fix #59724

1. Remove quotes from $(PYTHON) that were defeating conditions checking for an empty value, move to point of use.
2. Add error message for missing Python.
3. Also add error for missing C++ components at a location I encountered.

Tested clr and mono on Windows and Linux with and without Python.
Tested clr with Windows before and after adding missing C++ bits.

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Issue Details

---

Fix #59724

1. Remove quotes from $(PYTHON) that were defeating conditions checking for an empty value, move to point of use.
2. Add error message for missing Python.
3. Also add error for missing C++ components at a location I encountered.

Tested clr and mono on Windows and Linux with and without Python.
Tested clr with Windows before and after adding missing C++ bits.

Author:	danmoseley
Assignees:	danmoseley
Labels:	area-Infrastructure
Milestone:	-

[danmoseley]

Before

/home/dan/git/runtime/eng/python.targets(22,5): error MSB3073: The command "..." exited with code 127. [/home/dan/git/runtime/src/coreclr/System.Private.CoreLib/System.Private.CoreLib.csproj]

After

/home/dan/git/runtime/eng/python.targets(22,5): error MSB3073: The command "..." exited with code 127. [/home/dan/git/runtime/src/coreclr/System.Private.CoreLib/System.Private.CoreLib.csproj]
/home/dan/git/runtime/eng/python.targets(34,7): error : Python not found. Please add Python 3 to your path and try again. [/home/dan/git/runtime/src/coreclr/System.Private.CoreLib/System.Private.CoreLib.csproj]

I could suppress the first error, but then it wouldn't handle the case where Python was found, failed, and wrote to stdout. I can't find a way to make that happen but this way you'd still get the first error in such a situation.

[danmoseley]

failure was #81249. this is a test failure, so it's downstream of build and this change is validated.