Improve code coverage of tests

[Thalhammer]

Doing code coverage as introduced in #76 shows there is a lack of testing for some parts of the codebase (especially error handling of rare cases).
This should get improved.

Related to this:

• Moving to std::error_code (Provide an error_code based interface #72)
• Removal of dead code
• Building of examples (Compile examples #75)

[prince-chrismc]

closed with #90