Before:
```
(lldb) type synthetic add -l blah foo
warning: The provided class does not exist - please define it before attempting to use this synthetic provider
```
After:
```
(lldb) type synthetic add -l blah foo
warning: The provided class 'blah' does not exist - please define it before attempting to use this synthetic provider
```
Useful when many of these registration commands happen as part of
`~/.lldbinit`. Previously it wasn't immediately obvious which of those
commands failed.
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