Report compiler bugs when encountered.

Related to https://github.com/evancz/Elm/issues/430.
Start it up and go to `localhost:8000/try` and enter
```haskell
main = Bool
```
To see the message:
```
Elm compiler bug found!
Please report this as an issue at https://github.com/evancz/Elm/issues including your elm code and the following error message:
"Could not find 'Bool' when solving type constraints."
```

Dup of https://github.com/evancz/elm-lang.org/pull/102 but targeting dev.