error validation + readme updated re: valid characters

Author: joeuhren

README.md

@@ -17,4 +17,7 @@ Decimal Address Prefix: 0
 The first argument of **1** is the address prefix for the Bitcoin network.  
 The 2nd argument of **BurnAddress** is the vanity keyword to insert.
 
+**NOTE:** Only certain alphanumeric characters are valid for the address prefix and vanity keyword. The list of valid characters are: **123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz**
+
 The actual address is **1BurnAddressXXXXXXXXXXXXXXXXV7cPS6** and as an added check, the decimal address prefix of **0** is also displayed to eliminate guessing. The decimal address prefix must match the 2nd bit in the **PUBKEY_ADDRESS** of the coin's source code or else the address will not be valid. [Click here to view the PUBKEY_ADDRESS for Bitcoin's mainnet](https://github.com/bitcoin/bitcoin/blob/257f750cd986641afe04316ed0b22b646b56b60b/src/chainparams.cpp#L132). If you are not able to generate a valid address for your network it is most likely because the decimal address prefix doesn't match. You may have to change the vanity keyword to something that starts with a different number or letter to find one that matches your network's decimal address prefix. [Click here for more information about how the address prefixes work](https://en.bitcoin.it/wiki/List_of_address_prefixes).
+

unspendable.py

@@ -112,7 +112,7 @@ def generate (prefix_string, vanity_keyword, prefix_bytes):
             break
         if valid == False:
           # Invalid character found
-          print("Error: The address prefix must only contain numers or letters.")
+          print("Error: Invalid characters detected in the address prefix. Valid characters are: " + b58_digits)
           cont = False
           break
     if cont == True:
@@ -126,7 +126,7 @@ def generate (prefix_string, vanity_keyword, prefix_bytes):
             break
         if valid == False:
           # Invalid character found
-          print("Error: The vanity keyword must only contain numers or letters.")
+          print("Error: Invalid characters detected in the vanity keyword. Valid characters are: " + b58_digits)
           cont = False
           break		  
     if cont == True: