[mlir][vector] Refine diagnostic messages

Clarify a few diagnostics so that they are more consistent with the
corresponding condition. For example:

```
  if (positionAttr.size() >
      static_cast<unsigned>(getSourceVectorType().getRank()))
```

should lead to ("no greater than"):

```
    return emitOpError(
        "expected position attribute of rank no greater than vector rank");

```

as opposed to ("smaller"):

```
    return emitOpError(
        "expected position attribute of rank smaller than vector rank");
```

Differential Revision: https://reviews.llvm.org/D154998