Provide meaningful response in case of IO or auth failure

[zero323]

The multiple points of failure here, some of which are likely to be encountered by the users:

• Incorrect API key, project or table name
• Networking issues

Depending on a function these can escalate to user code (where standard reader functions, like read.csv or fromJSON are used) or be suppressed (when httr is used).

From user perspective it could nice if these were handled consistently and resulted in meaningful exception and / or error messages.