Error handling improvements

[tbitonti]

Ref:

/org.eclipse.transformer/src/main/java/org/eclipse/transformer/Transformer.java

// TODO: Need a better way to handle errors. // // (1) Having element actions throw an exception while // having container actions count failures makes for // two different error detection steps. // // (2) Handling 'Throwable' cases is problematic. These // should perhaps be thrown to the transformer and cause // transformation to fail.