Throwing a proper RuntimeException when a property could not be loaded f...

...or a client because it didn't have a property name after the namespace

The load method throws a ```java.lang.StringIndexOutOfBoundsException``` currently for properties like this -

myclient.ribbon.=baz
myclient.ribbon=bar

This PR throws a ```java.lang.RuntimeException``` with the name of the property which is invalid in the message of the exception instead of the ```java.lang.StringIndexOutOfBoundsException```

We could also just log an error and load rest of the properties but this would change the current behavior where the method returns after throwing the ```java.lang.StringIndexOutOfBoundsException``` which also happens to be a subclass of ```java.lang.RuntimeException```
Should the argument be 'key' rather than 'prop'?  Put another way, will users be confused by having the namespace omitted in the error message.  Alternatively, should the namespace be included as a separate value in the message, for example: ```String.format("Property '%s' in namespace '%s' is invalid.", prop, getNameSpace())```