Customize the macro error message based on the metadata

[Nemo157]

fixes #2767

[abonander]

In the case of SQLX_OFFLINE=true, the wording is rather confusing.

I'd prefer something like:

Offline mode is forced on because SQLX_OFFLINE=true is set in the environment variables, but there is no cached data for this query. Please run cargo sqlx prepare to update the offline query data cache, or unset SQLX_OFFLINE.

[Nemo157]

I reduced the start of the message a bit, if someone has explicitly set SQLX_OFFLINE I assume they know what it is. I also reduced the variation in messages that I'd added, now it's simply one message for offline and one for online-without-database-url.