[ Refactoring / Code Improvements ] Incorporate naming and typing feedback from PR 477

[kartg]

This issue tracks open action items from #477 based on PR feedback from @gregschohn :

• Rename index_operations to something that's more clear

Index is both a noun and a verb, so it isn't clear to me whether this applies to operations on indices (nouns) or if this is a set of operations of a particular type (verb). index_management.py, or something that could shake the ambiguity would probably help new contributors.

• Use custom errors across the code base rather than a chain of RuntimeError

This stems from #477 (comment)