message: error if unable to calculate BUILD_VERSION
diff: @@ -157,15 +157,18 @@ if(BUILD_VERSION STREQUAL "0.0.0")
  )
  if(NOT CALC_RELEASE_VERSION_RESULT STREQUAL 0)
  # If python failed above, stderr would tell the user about it
  - message(FATAL_ERROR "BUILD_VERSION not specified and could not be calculated")
  + message(FATAL_ERROR
  + "BUILD_VERSION not specified and could not be calculated\
  + (script invocation failed); specify in CMake command, -DBUILD_VERSION=<version>"
  + )
  else()
  set(BUILD_VERSION ${CALC_RELEASE_VERSION})
  file(WRITE ${CMAKE_BINARY_DIR}/VERSION_CURRENT ${CALC_RELEASE_VERSION})
  endif()
  else()
  - message(WARNING
  + message(FATAL_ERROR
  "BUILD_VERSION not specified and could not be calculated\
  - (Python was not found on the system); setting library version to 0.0.0"
  + (Python was not found on the system); specify in CMake command, -DBUILD_VERSION=<version>"
  )
  endif()
  endif()
  