Explorer does not handle invalid transaction hashes

[jpraynaud]

Why

When trying to verify a transaction hash which is invalid with the explorer, an error message appears: Application error: a client-side exception has occurred (see the browser console for more information).

What

Handle invalid transaction hashes properly in a modal with a clear error message.

How

• Fix explorer to handle invalid transaction hashes
• Restrict the verification of the transactions before calling the aggregator with a regexp that accepts only 64 hexadecimal characters