Improve error messages when embedSdf.py fails

Check embedSdf.py return code and print stderr on failure.
Partial backport of #1549.

Signed-off-by: Steve Peters <scpeters@openrobotics.org>

sdf/CMakeLists.txt

@@ -19,11 +19,18 @@ endif()
 
 # Generate the EmbeddedSdf.cc file, which contains all the supported SDF
 # descriptions in a map of strings. The parser.cc file uses EmbeddedSdf.hh.
+set(EMBEDDED_SDF_CC_PATH "${PROJECT_BINARY_DIR}/src/EmbeddedSdf.cc")
 execute_process(
   COMMAND ${Python3_EXECUTABLE} ${CMAKE_SOURCE_DIR}/sdf/embedSdf.py
-    --output-file "${PROJECT_BINARY_DIR}/src/EmbeddedSdf.cc"
+    --output-file "${EMBEDDED_SDF_CC_PATH}"
   WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}/sdf"
+  RESULT_VARIABLE EMBEDDED_SDF_RESULT
+  ERROR_VARIABLE EMBEDDED_SDF_ERROR
 )
+# check process return code
+if(NOT EMBEDDED_SDF_RESULT EQUAL 0)
+  message(FATAL_ERROR "Error executing ${Python3_EXECUTABLE} ${CMAKE_SOURCE_DIR}/sdf/embedSdf.py to create ${EMBEDDED_SDF_CC_PATH}: ${EMBEDDED_SDF_ERROR}")
+endif()
 
 # Generate aggregated SDF description files for use by the sdformat.org
 # website. If the description files change, the generated full*.sdf files need