Emit better warning. (#2071)

Before
```
✦2 ❯ modcc -t cpu jnml-arbor/SKv3_1.mod
Warnings:
  * jnml-arbor/SKv3_1.mod:(line 1,col 1) Assignments to local variable containing state variables will not be integrated in time
```
After
```
✦2 ❯ modcc -t cpu jnml-arbor/SKv3_1.mod
Warnings:
  * jnml-arbor/SKv3_1.mod:(line 172,col 14) Assignments to local variable containing state variables will not be integrated in time: (rate_m_q = ((m_inf - m_q) / m_tau))
```