kunit: tool: revamp message for invalid kunitconfig

The current error message is precise, but not very clear if you don't
already know what it's talking about, e.g.

> $ make ARCH=um olddefconfig O=.kunit
> ERROR:root:Provided Kconfig is not contained in validated .config. Following fields found in kunitconfig, but not in .config: CONFIG_DRM=y

Try to reword the error message so that it's
* your missing options usually have unsatisified dependencies
* if you're on UML, that might be the cause (it is, in this example)

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>