The async worker that drains the engine queue catches `Exception` with
no `as` binding and prints the bare line
"A error occurred while processing a 'work item'."
That message both swallows the actual error class+text (so users
report bugs like #2227 with no actionable detail and the maintainer
has to ask them to repro under more verbosity) and reads "A error"
instead of the grammatical "An error".
Bind the exception, print its type and message, and fix the article.
The shodan integration test (`tests/discovery/test_shodan_engine.py`)
already asserts the message is not in output on the happy path; the
assertion is updated to match the new string.
Closes#2227.
L1ghtn1ng