cmake: improved DTC detection handling

Fixes: #26096

In some cases DTC may be installed, but in a non-working state.
In such cases, CMake will abort processing with an cryptic error.

This commit now handles dtc version errors and print a warning., and
CMake processing continues.

dtc is an optional component and failures should not break CMake run.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>