Improved error messages.

[aleator]

This patch improves some of the error messages, but I didn't manage to make a really reasonable progress before running out of time.

1. Bad field names on record completion:

echo "{Type =  {x : Integer, y : Bool}, Default = {y = True}}::{x = 5}" |stack exec dhall

Use "dhall --explain" for detailed errors

Error: Completion record is missing a field: default

1│ {Type =  {x : Integer, y : Bool}, Default = {y = True}}::{x = 5}

(stdin):1:1

2. Trying to complete non-records:

Use "dhall --explain" for detailed errors

Error: You can only complete records

1│ True::{x = 5}

(stdin):1:1

3. Possibly better short message on if predicates

echo "if 1 then 2 else 3" |stack exec dhall

Use "dhall --explain" for detailed errors

Error: Invalid predicate for ❰if❱: Natural

1│ if 1 then 2 else 3

(stdin):1:1

4. Possibly better short message on list annotations. This is probably
   superfluous

echo "[] : Bool" |stack exec dhall

Use "dhall --explain" for detailed errors

Error: Invalid type for ❰List❱: Bool

1│ [] : Bool

5. Better short message on missing constructors:

echo "<Foo : Bool>.Boo True" |stack exec dhall

Use "dhall --explain" for detailed errors

Error: Missing constructor: Boo

1│ <Foo : Bool>.Boo

(stdin):1:1

6. Better short messages on missing handlers

[sjakobi]

Very nice improvements!

[Gabriella439]

Looks great to me! Just one minor comment