Fix address format error and tighten up tests

src/index.js

@@ -36,7 +36,7 @@ class Vain extends Emitter {
 
 		this.options.prefix.split('').forEach(char => {
 			if (!this.addressFormat.charset.includes(char)) {
-				throw new Error(`Invalid characters for address format "${this.options.prefix.addressFormat}"`);
+				throw new Error(`Invalid characters for address format "${this.options.addressFormat}"`);
 			}
 		});
 

test/errors.js

@@ -31,7 +31,7 @@ const multisigOptions = {
 invalidCharsets.forEach(options => {
 	test(`Vain throws when \`options.prefix\` doesn't match the charset for ${options.addressFormat} addresses`, t => {
 		const error = t.throws(() => new Vain(options));
-		t.true(error.message.includes('Invalid characters for address format'));
+		t.is(error.message, `Invalid characters for address format "${options.addressFormat}"`);
 	});
 });