[mlir:Parser] Emit a better diagnostic when a custom operation is unknown

When a custom operation is unknown and does not have a dialect prefix, we currently
emit an error using the name of the operation with the default dialect prefix. This
leads to a confusing error message, especially when operations get moved between dialects.
For example, `func` was recently moved out of `builtin` and to the `func` dialect. The current
error message we get is:

```
func @foo()
^ custom op 'builtin.func' is unknown
```

This could lead users to believe that there is supposed to be a `builtin.func`,
because there used to be. This commit adds a better error message that does
not assume that the operation is supposed to be in the default dialect:

```
func @foo()
^ custom op 'func' is unknown (tried 'builtin.func' as well)
```

Differential Revision: https://reviews.llvm.org/D125351