clarify connection-related error messages

[davepacheco]

While doing oxidecomputer/omicron#3232 I found the error message for connection-related errors always says that it was checking out a connection, but that isn't necessarily true. As far as I can tell, this variant is used in two places:

• during actual connection establishment, when that fails: https://github.com/oxidecomputer/async-bb8-diesel/blob/master/src/connection_manager.rs#L83
• when the pool actively checks whether a connection is valid: https://github.com/oxidecomputer/async-bb8-diesel/blob/master/src/connection_manager.rs#L89

I'm changing the variant name and error message here to just say that this was a connection-related error.

[davepacheco]

See oxidecomputer/omicron#3233 for examples.