Add postflight to handle click exc and exit codes

[stu-k]

resolves #7010

Description

• Added a postflight decorator to house any actions necessary after an invocation has been completed
• Moved existing firing of the CommandComplete event to postflight
• Added exception wrapping in postflight to allow click to surface the correct error codes
• Added additional exception handling in dbtRunner to handle exceptions thrown by postflight

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[ChenyuLInx]

Looks good to me! Thanks for finding this solution! I was able to validate the cli exit code 0,1,2 when running in a terminal(using echo $?). I am wondering is it possible to add the test to validate that code(not the exception thrown). Not a blocker if not!