escalating missing library to FATAL_ERROR

It was previously a WARNING in CMake, but that
leads to missing symbol errors, which can be
misleading since the library was actually not
found but the first inclination is to check the
library which contains the symbols for errors.

We might consider the need to change this back
in the future for cases where having the library
is not critical.

ament_cmake_export_libraries/cmake/ament_cmake_export_libraries-extras.cmake.in

@@ -39,7 +39,7 @@ if(NOT "${_exported_library_targets} " STREQUAL " ")
 
     if(NOT _lib)
       # warn about not existing library and ignore it
-      message(WARNING "Package '@PROJECT_NAME@' exports the library '${_library_target}' which couldn't be found")
+      message(FATAL_ERROR "Package '@PROJECT_NAME@' exports the library '${_library_target}' which couldn't be found")
     elseif(NOT IS_ABSOLUTE "${_lib}")
       # the found library must be an absolute path
       message(FATAL_ERROR "Package '@PROJECT_NAME@' found the library '${_library_target}' at '${_lib}' which is not an absolute path")