Unify Appender error handling

[taniabogatsch]

This is the first follow-up PR to #187.

It focuses on unifying the error handling in the Appender. Additionally, errors in the Appender no longer kill the application (panic) and have a unified format.

I'm happy that even with the introduction of new tests, this PR decreases the overall code size of the Appender and its tests. 😄 Sorry for the big PR - I've tried to keep the complexity low. Many changes are repetitive (using new functions) and structural.

Other changes

• unified variable name refactoring according to their type definitions and methods (e.g., connector -> c, appender -> a, appender *C.duckdb_appender -> duckdbAppender C.duckdb_appender).
• added missing C.duckdb_appender_destroy(&duckdbAppender) when the appender's column type initialization fails to avoid leaks.
• removed duplicate code in appender_test.go by introducing a new function: cleanupAppender.
• moved error-tests from appender_test.go to errors_test.go.