Add more checks of function return values

Software analysis pointed some questionable source code places out.

• The tool “clang-tidy” informed about the implementation detail “Value stored to 'ret' is never read”.
• Some return values were not checked so far from calls of functions according to the SQLite programming interface. This issue was detected also by using the software “Coccinelle 1.0.8”.

Thus adjust affected places accordingly.

• Increase error detection and corresponding exception handling.
• Improve transaction management.
• Omit a few initialisations of local variables.
• Optimise a few functions a bit more.