chore: amend a log message which could cause users to be panic

[yangby-cryptape]

For example, the 8~10 lines of the log file for our all integration tests at present are:

Failed to open AddrManager db, file: "XXX/addr_manager.db", error: Os { code: 2, kind: NotFound, message: "No such file or directory" }
Failed to open BanList db, file: "XXX/ban_list.db", error: Os { code: 2, kind: NotFound, message: "No such file or directory" }
receiving shutdown signal ...

I think it's a bit confusing and scary.

[doitian]

bors r=doitian,TheWaWaR

[bors]

Build succeeded:

• continuous-integration/travis-ci/push