Remove robot not found error when parsing fails

[marcoag]

🦟 Bug fix

Fixes #151

Summary

When a file fails to be parsed as SDF the parser.cc tries to parse it as URDF generating the following error:

Error:   Could not find the 'robot' element in the xml file
             at line 109 in ./urdf_parser/src/model.cpp

This becomes confusing the file could just be a malformed SDF.

This PR adds a check for the robot tag before attempting the parse in order to avoid the misleading error. It also adds an error in case it's not found indicating that the file could not be identified as SDF nor URDF.

NOTE: I'm targeting sdf13 as I consider this a bug fix but let me know if this should go into master.

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.

[codecov]

Codecov Report

Merging #1290 (7ac91df) into sdf13 (a2eb081) will decrease coverage by 0.08%.
The diff coverage is 81.25%.

❗ Current head 7ac91df differs from pull request most recent head b4c995f. Consider uploading reports for the commit b4c995f to get more accurate results

@@            Coverage Diff             @@
##            sdf13    #1290      +/-   ##
==========================================
- Coverage   87.67%   87.59%   -0.08%     
==========================================
  Files         128      128              
  Lines       16814    16826      +12     
==========================================
- Hits        14741    14739       -2     
- Misses       2073     2087      +14     

Impacted Files	Coverage Δ
src/parser.cc	86.80% <81.25%> (-0.37%)	⬇️

... and 1 file with indirect coverage changes

[azeey]

This is a much needed UX improvement, so thanks!! I've left some minor comments, but overall it looks great! Would you be able to add some tests?

[marcoag]

When adding the tests I realized that we need to check for the <sdf> tag and ONLY if not present then we could print the "nor SDFormat nor URDF" fail. Otherwise the error will show up on malformed SDFormat files and it gets confusing.

Added relevant changes in 029b204.

[marcoag]

a1a3ba9 adds one more test that checks the Could not find the 'robot' element in the xml file error is not printed when parsing a malformed SDF file.