Updates Java UserAgent Error Handling

A concern was raised over the possibility of an Error being thrown during the
initialization of the UserAgent class resulting in the class to fail to load
and giving users an obscure NoClassDefFoundError. I'm upgrading the existing
catch(Exception)'s to catch(Throwable)'s to catch these potential Error's and
ensure that the user agent class will always load correctly. Any missing data
due to caught errors will be replaced with a default value of "NotAvailable"

Missing data in the user agent on rare occasions is a small concern and preferable
to the driver crashing altogether.

Author: Cole-Greer

gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/util/Gremlin.java

@@ -32,7 +32,7 @@ public final class Gremlin {
         try {
             version = Manifests.read("version");
         }
-        catch (Exception e) {
+        catch (Throwable t) {
             version = "VersionNotFound";
         }
     }

gremlin-driver/src/main/java/org/apache/tinkerpop/gremlin/driver/UserAgent.java

@@ -59,7 +59,7 @@ private static String getAttributeOrDefault(ThrowingSupplier<String> supplier){
         try {
             ret = supplier.get().replace(' ', '_');
         }
-        catch (Exception e) {
+        catch (Throwable t) {
             // No action taken, default value of "NotAvailable" will be used if supplier fails
         }
         return ret;