message: [cmake] Print out errors from failed stack steps
  Per [cmake docs](https://cmake.org/cmake/help/v3.0/command/execute_process.html), if you set both `OUTPUT_VARIABLE` and `ERROR_VARIABLE` to the same var, it will contain both streams' lines, merged in the order they were printed.
diff: @@ -14,11 +14,11 @@ execute_process (
  COMMAND ${STACK_EXECUTABLE} ${BOND_STACK_OPTIONS} setup
  WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
  RESULT_VARIABLE error
  - OUTPUT_QUIET
  - ERROR_QUIET)
  + OUTPUT_VARIABLE setup_output
  + ERROR_VARIABLE setup_output)
  if (error)
  - message (FATAL_ERROR)
  + message (FATAL_ERROR ${setup_output})
  endif()
  set (buildGhcOptions "-O2")
  @@ -29,7 +29,7 @@ execute_process (
  RESULT_VARIABLE error)
  if (error)
  - message (FATAL_ERROR)
  + message (FATAL_ERROR "build failed - scroll up for its output")
  endif()
  # Copy results to builddir
  @@ -37,10 +37,11 @@ execute_process (
  COMMAND ${STACK_EXECUTABLE} ${BOND_STACK_OPTIONS} path --dist-dir
  WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
  OUTPUT_VARIABLE dist_dir
  - RESULT_VARIABLE error)
  + RESULT_VARIABLE error
  + ERROR_VARIABLE path_errors)
  if (error)
  - message (FATAL_ERROR)
  + message (FATAL_ERROR ${path_errors})
  endif()
  string (STRIP ${dist_dir} dist_dir)
  