Improve expression type checking error for "match"

Nobody has claimed this yet.

Using the following expression as a filter (using branch https://github.com/mapbox/mapbox-gl-js/pull/5193 at the time of writing):

```
"filter": {
"expression": [
"curve",
["step"],
["zoom"],
7,
"all",
8,
[
"<", [ "number", [ "get", "area" ] ], 80000
],
9,
[
"<", [ "number", [ "get", "area" ] ], 20000
]
]
}
```

Produces the error, `filter.expression[6]: Expected String but found Boolean instead.`.

Ideally, the error would have been: `filter.expression[4]: Expected Boolean but found String instead`
With perhaps a secondary explanation: `filter expressions must produce Boolean results`
Even better `filter.expression[4]: Expected Boolean but found String literal "all" instead`

(h/t @aparlato @jfirebaugh )

Start by reproducing the reported filter expression and its current error, then trace the expression type-checking path for the "match" case. Done means the diagnostic points to expression index 4, reports the expected Boolean versus the found String literal "all", and may explain that filter expressions must produce Boolean results.

Written by the indexing model from the issue text.