Don't thrown an exception if the JAR manifest does not specify the client version

If the Stormpath client classes are embedded within another JAR (e.g., using the Maven shade plugin) and the original manifest file is not included, the client version lookup heuristic fails with an error. It seems that this version string is purely informational, given that lookupClientVersion() returns "NOT-FROM-JAR" under similar circumstances.

I have no opinion on the correct string to return in such cases, but throwing an exception seems too severe.