Those really aren't good titles, and just considering the major tag doesn't help them a whole lot. They need to be edited for de-duplication.
You're absolutely right that error messages should be present verbatim in a question because they're good (even optimal) search targets, but there still needs to be more information than that in the title.
When you come up with six different "CatVomitError: you will need a new couch" questions in your Google results, the major tag is already prepended to the titles, so that information is available to searchers. The thing is, even error message plus language/framework is not particularly definitive -- there's usually a few different ways to get the same message, meaning you have to click through anyways to see if your exact situation matches.
The OP (or an editor with more expertise in the subject) should be able to add something along the lines of "...when trying to stir the oatmeal", or "...but I'm not using a garden hose", or "...even though the red button was already pushed" which will a) instantly make the title unique, and b) make it way more useful to searchers.
In short, the tag doesn't make the title much more specific, which is the goal of the prohibition on duplicate titles. The title needs some meat to it other than one more technical word, and leaving the restriction as it is furthers that goal.