Fix prob_cleanup

[jgosmann]

Motivation and context:

This fixes the documentation where one statement was wrong. Also, an exception is raised instead of returning incorrect values, when the calculation fails due to limited floating point precision.

Fixes #203.

Interactions with other PRs:

none

How has this been tested?

added assert in the relevant test

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Where should a reviewer start?

Please make sure that the documentation is actually correct and that I didn't confuse myself in the end.

Types of changes:

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• I have run the test suite locally and all tests passed.