Standardize error message formatting

[julianrkung]

Maybe this is something we can just address all at once later, but it might be good to standardize our error message formatting since we're using pretty different things throughout e.g.

(Meter Reading) Error occurred when creating meter reading:

(createPurchaseRequestAndUpdateInventory) Error:

An error occurred while updating the customer:

Originally posted by @wangannie in #99 (comment)