Replaced "0000000000" with exceptions

Fixes #252. Passing invalid input to the constructor should now expect
an IllegalArgumentException.

exercises/phone-number/src/example/java/PhoneNumber.java

@@ -16,8 +16,12 @@ public class PhoneNumber {
         }
 
         final boolean numberIsValid = (number.length() == 10);
-        final String normalizedNumber = numberIsValid ? number : "0000000000";
-        return normalizedNumber;
+
+        if(!numberIsValid) {
+            throw new IllegalArgumentException();
+        }
+        
+        return number;
     }
 
     public String getNumber() {

exercises/phone-number/src/test/java/PhoneNumberTest.java

@@ -39,25 +39,15 @@ public class PhoneNumberTest {
     }
 
     @Ignore
-    @Test
+    @Test(expected = IllegalArgumentException.class)
     public void invalidWhenOnly11Digits() {
-        final String expectedNumber = "0000000000";
         final String actualNumber = new PhoneNumber("21234567890").getNumber();
-
-        assertEquals(
-            expectedNumber, actualNumber
-        );
     }
 
     @Ignore
-    @Test
+    @Test(expected = IllegalArgumentException.class)
     public void invalidWhen9Digits() {
-        final String expectedNumber = "0000000000";
         final String actualNumber = new PhoneNumber("123456789").getNumber();
-
-        assertEquals(
-            expectedNumber, actualNumber
-        );
     }
 
     @Ignore