Make "Invalid DSN" errors consistent in Syncstorage-rs

[data-sync-user]

As we add datastores to syncstorage, we should make the DSN parse errors more consistent. Right now, we display Spanner or SQLite database_url specified for MySQL feature, please correct. or whatever. Instead we should just use something like Invalid DSN specified for {} feature, please correct: {} where we specify the feature type and the invalid DSN.

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