Categories: Suppress error message when trying to add an existing category.

A user suggested an enhancement while adding categories that actually meets the current behavior in the iOS app. In their case, the site has hundreds of categories, which makes it difficult to scroll through many pages and select the desired ones.

Requested here: 5608189-zd-woothemes

Expected behavior

When trying to create a category that already exists, instead of displaying an error message, the app would either:

1. Apply the existing category, in case it was not yet applied (Current behavior on iOS);
2. Ignore the error, in case the category was already applied (Current behavior on iOS).

Actual behavior

When trying to create a category that already exists, a toast with the message Adding category failed is displayed and nothing happens with the category.

Steps to reproduce the behavior

1. Create a new post
2. Post settings > Categories
3. Try to create a category that already exists

Reproduce the issue from a new post through Post settings > Categories, then locate the Android category-creation flow. Compare the existing-category outcomes described for iOS: apply the category if needed and otherwise ignore the duplicate; done means no “Adding category failed” toast and the expected category state is preserved.