Add option to pass custom error handler
Add option to pass custom error handler

Refs #14

rubenstolk/IDBWrapper,GerHobbelt/IDBWrapper,jensarps/IDBWrapper,jensarps/IDBWrapper,kilbot/IDBWrapper,safetychanger/IDBWrapper,mizchi/idb-wrapper-promisify,safetychanger/IDBWrapper,jayfunk/IDBWrapper,mizchi/idb-wrapper-promisify,rubenstolk/IDBWrapper,kilbot/IDBWrapper,jayfunk/IDBWrapper