Solve -Wreturn-type compiler warnings

Description

From commit message:

Some functions that should have returned values had code paths that
didn't, which is undefined behavior. Make sure that we always throw or
return something appropriate.

Related Issue

#19 (closed)

How Has This Been Tested?

Compiles. Code paths are most likely unused (90% sure it would have crashed).

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.
• I have added tests to cover my changes.
• All new and existing tests passed.