Fixes to remove warnings for PXR_STRICT_BUILD_MODE

[pmolodo]

Description (this won't be part of the changelog)

Just some small tweak to get rid of build warnings, so you can build with PXR_STRICT_BUILD_MODE

Changelog

Fixed

• Fixed warning about signed/unsigned comparsion
• Fixed warning about unused errorString

Checklist (Please do not remove this line)

• [X] Make sure the Title and Description of the PR make sense and provide sufficient context for your work
• [X] Do any added files have the correct AL Apache Licence Header?
• [X] Are there Doxygen comments in the headers?
• [X] Are any new features, behaviour changes documented in the .md format documentation?
• [X] Have you added, updated tests to cover new features and behaviour changes?
• [X] Have you filled out at least one changelog entry?

[pmolodo]

Rebased this against the latest develop (since you already fixed one of the warnings!)