Making error messages clearer

[AtilaSaraiva]

I propose this issue as a venue to report error messages that weren't helpful in the past, you or that they didn't fit your usecase. This will most likely spawn, or so I hope, a series of commits to improve the error messages for the users.

I will start with my own:

I was working on something on devito, and got stuck up to an hour trying to figure out that this error:

ValueError: Unrecognized argument damp=damp(x, y).

meant the operator expressions didn't have the function damp. Thinking back it is obvious but the error message could have been cleaerer. I'm just making this commit as way to prevent another person to go through this. If I encounter other situations such as this I will open another PR so that we can make the error messages and error catching mechanisms work better.

I hope there will be a collective effort to make the Devito framework more acessible, with a less steep learning curve.