Fail better in the CLI when using the ChecksumAddress type

Also, clarify that account selection prompt is based on index

nucypher/cli/actions.py

@@ -449,7 +449,7 @@ def select_client_account(emitter,
     emitter.echo(tabulate(rows, headers=headers, showindex='always'))
 
     # Prompt the user for selection, and return
-    prompt = prompt or "Select Account"
+    prompt = prompt or "Select index of account"
     account_range = click.IntRange(min=0, max=len(enumerated_accounts)-1)
     choice = click.prompt(prompt, type=account_range, default=default)
     chosen_account = enumerated_accounts[choice]
@@ -485,7 +485,7 @@ def handle_client_account_for_staking(emitter,
         if staking_address:
             client_account = staking_address
         else:
-            client_account = select_client_account(prompt="Select staking account",
+            client_account = select_client_account(prompt="Select index of staking account",
                                                    emitter=emitter,
                                                    registry=stakeholder.registry,
                                                    network=stakeholder.network,

nucypher/cli/types.py

@@ -27,7 +27,10 @@ class ChecksumAddress(click.ParamType):
     name = 'checksum_address'
 
     def convert(self, value, param, ctx):
-        return to_checksum_address(value=value)  # TODO: More robust validation here?
+        try:
+            return to_checksum_address(value=value)
+        except ValueError as e:
+            self.fail(str(e))
 
 
 class IPv4Address(click.ParamType):