Add await when processing header to avoid unhandleRejection

[deanhannigan]

There was an await missing in the call to the _processHeaderRow function that would cause an unhandledRejection error when any error was encountered. The side effect of this is that the unhandledRejection error goes under the radar and results in a uncaughtException error that kills the Budibase server.

To reproduce

• Create a Google sheet. Ensure at least one of more of the sheets have no headers.
• Add the sheet to your datasources in Budibase.
• Click Fetch tables for Google sheet. Select all the sheets.
• Click Fetch Sheets. The import will fail and the server will crash with an unhandledRejection

Our koa app also needs a modification to better handle the unhandledRejection