Assigning electors fails silently when elector has invalid account address

[mazito]

When assigning electors it may happen that the MINA account address that has been set in the user Profile is invalid for some reason.
It maybe that some char is missing ,etc.
In this case, the assignment of electors fails:

• The server gives and Error 500
• Thi is not used in the App UI and does not provide any feedback to the user.

MUST fix and give adecuate feedback when this happens, ideally:

• Why it failed
• Elector with the problematic account address

[mazito]

Idea: verify that all electors have a valid account BEFORE doing the assignment pŕocess.