Improve error messages when embedSdf.py fails

[scpeters]

🦟 Bug fix

Follow-up to #1536.

Summary

The embedSdf.py script is used to generate the EmbeddedSdf.cc source code that implements the interface defined in src/EmbeddedSdf.hh. The contribution by @ efferre79 in #1536 adds error checking to confirm that the generated EmbeddedSdf.cc file is not empty. This pull request makes some additional improvements to this error checking.

a44ccb2: check that EmbeddedSdf.cc exists before checking its size, use string comparisons, and rephrase the error message

Currently the file(SIZE) call generates a cmake error if the EmbeddedSdf.cc file does not exist and the if(${OUT_SIZE} EQUAL 0) comparison fails if OUT_SIZE is an empty variable:

CMake Error at sdf/CMakeLists.txt:28 (file):
  file SIZE requested of path that is not readable:

    <CMAKE_BINARY_DIR>/src/EmbeddedSdf.cc


CMake Error at sdf/CMakeLists.txt:29 (if):
  if given arguments:

    "EQUAL" "0"

  Unknown arguments specified

After a44ccb2, the error message when EmbeddedSdf.cc is empty or does not exist changes to:

CMake Error at sdf/CMakeLists.txt:34 (message):
  <CMAKE_BINARY_DIR>/src/EmbeddedSdf.cc is
  empty or does not exist

1ca9bb6: check return code of embedSdf.py and print stderr

This checks the RESULT_VARIABLE of the execute_process call and prints the ERROR_VARIABLE if it is not successful. After this change, the error message is:

CMake Error at sdf/CMakeLists.txt:33 (message):
  Error executing
  /usr/local/Frameworks/Python.framework/Versions/3.13/bin/python3.13
  /Users/scpeters/ws/sdformat_math/src/sdformat/sdf/embedSdf.py to create
  /Users/scpeters/ws/sdformat_math/src/sdformat/build/src/EmbeddedSdf.cc:
  Traceback (most recent call last):

    File "/Users/scpeters/ws/sdformat_math/src/sdformat/sdf/embedSdf.py", line 13, in <module>
      intentional_error

  NameError: name 'intentional_error' is not defined

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[efferre79]

Looks good to me!

[scpeters]

we can backport 1ca9bb6, since that should work with older cmake versions

[scpeters]

we can backport 1ca9bb6, since that should work with older cmake versions

partial backport in #1550