cmake: toolchain: Improve error feedback when toolchain is not found

Improve the error feedback when the toolchain is not found.

Currently, for some setups, presumably primarily SDK-less setups,
users can get an error message like this for certain types of
misconfigurations:

CMake Error at zephyr/cmake/extensions.cmake:984 (message):
  No such file or directory: CMAKE_READELF: 'CMAKE_READELF-NOTFOUND'
Call Stack (most recent call first):
  zephyr/cmake/compiler/gcc.cmake:14 (assert_exists)
  zephyr/cmake/toolchain.cmake:38 (include)
  zephyr/cmake/app/boilerplate.cmake:243 (include)
  CMakeLists.txt:1 (include)

This commit will improve the error feedback to give a explanation for
what kind of failure has occured and will also dump relevant variables
to ease debugging.

This fixes #7075

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>