Invite error msg enhancement & fromWei augmentation

[Fang-]

Two unrelated commits:

• e88f47e makes sure we display the relevant email address when we tell the user it has already received an invite before. Note that receiving multiples of these, as is now possible, prints as a single-line concatenated string. The form validator should be printing these errors on their own lines.
• I still have not been able to reproduce Error Report: Please pass numbers as strings or BigNumber objects to avoid precision errors. #373 and friends locally. One report indicated this happening specifically with a higher amount of to-send invites, but this works fine when I test it. c53aba4 nevertheless wraps all fromWei and toWei calls in a function that does the toBN, and includes additional information in the error report in case of unexpected failure. If the issue continues to rear its ugly head, this should give us some more details to work with. I'll also make sure to deploy a build with non-minimized function names.

[liam-fitzgerald]

LGTM!