Better error message from integrate

<div id="comment:0"></div>

Of course the input is nonsense but the answer needs experience to understand.

```
integrate(x,1,1,2)
...
AttributeError: 'sage.rings.integer.Integer' object has no attribute 'variables'
s}}}

```

Component: **symbolics**

_Issue created by migration from https://trac.sagemath.org/ticket/21905_

