[3.x] Improve error message when OS.execute() fails on Windows

[Calinou]

This takes the error message from 4.x and backports it to 3.x.

PS: I noticed we don't have any such message when OS.execute() fails on Linux (and possibly macOS). This also applies to 4.x.

Testing project: test_os_execute_3.x.zip

• See [TRACKER] Error macros with cryptic messages due to not having proper custom messages defined #42719 (comment).

Preview

Before

ERROR: Condition "ret == 0" is true. Returned: ERR_CANT_FORK
   at: execute (platform/windows/os_windows.cpp:2981)
ERROR: Condition "ret == 0" is true. Returned: ERR_CANT_FORK
   at: execute (platform/windows/os_windows.cpp:2981)

After

ERROR: Could not create child process: non-existent-command
   at: (platform/windows/os_windows.cpp:2981)
ERROR: Could not create child process: non-existent-command with arguments
   at: (platform/windows/os_windows.cpp:2981)

[lawnjelly]

Thanks!