Adding helpful info to keyerror for StepResults.get_material

[shimwell]

Description

Inspired by this user story

This PR proposes adding to the error message that a user gets when trying to access a material id in the StepResult.get_material() that is not present.

import openmc
import openmc.deplete
results = openmc.deplete.Results.from_hdf5('depletion_results.h5')
step=results[0]
step.get_material('42')

current behaviour

>>> KeyError: '42'

proposed behaviour

KeyError: "mat_id '42' not found in StepResult. Available mat_id values are ['1', '2', '3']"

Checklist

• I have performed a self-review of my own code
  - [ ] I have run clang-format on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
  - [ ] I have added tests that prove my fix is effective or that my feature works (if applicable)<\s>

[paulromano]

Thanks @shimwell!