Fixed copy paste error in JS and cleaned up error handling in Java

Hi there,

there were two issues in the code:

1. Copy paste error in the JS file: the second method name should be `"paste"` not `"copy"`.
2. `success()` would be called outside the `try/catch` clause and after sending `error()` - moved into the `try` block.

I've also made some changes for my use:

3. Changed `equals()` calls to be called on the String constants instead of on the String argument. Avoids NPE.
4. `CallbackContext.error()` and `success()` methods are only convenience methods that create plugin results. Therefore, I replaced the creation of the PluginResult with status OK with `success(str)`.
5. I like informative error messages. Therefore, I'm using `PluginResult.Status` to inform on the different kinds of errors and add the Exception message if available.