representation.py: better assertion errors

[kevinfoley]

Description

Replaced two simple asserts with detailed error messages about what's gone wrong.

Related Issue

#353

Motivation and Context

I was working on a robot with 131 links and got an assertion error from representation.py, on this line:

assert self.input_type in MESH_DATA_TYPES

The generic assertion error and call stack was not helpful for understanding what went wrong or which of the 131 links was causing the problem. I had to look at the source code to better understand the problem, and then replace the simple assert with a more detailed AssertionError so I would know which link was causing the error.

While I was at it, I also added a more detailed AssertionError for the case where the meshname is None or not a string.

How Has This Been Tested?

I used the new AssertionError for self.input_type to identify which link in the robot was causing the issue.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.

[hwiedPro]

Thanks, that's a great contribution!