Improve Error messages

[ktec]

Currently the error messages look something like this:

iex> AgeCheck.user_older_than?("foobar", 42)
** (TypeCheck.TypeError) At lib/type_check_example.ex:28:
The call to `user_older_than?/2` failed,
because parameter no. 1 does not adhere to the spec `%User{age: integer(), name: binary()}`.
Rather, its value is: `"foobar"`.
Details:
  The call `user_older_than?("foobar", 42)` 
  does not adhere to spec `user_older_than?(%User{age: integer(), name: binary()},  integer()) :: boolean()`. Reason:
    parameter no. 1:
      `"foobar"` does not check against `%User{age: integer(), name: binary()}`. Reason:
        `"foobar"` is not a map.
    (type_check_example 0.1.0) lib/type_check_example.ex:28: AgeCheck.user_older_than?/2

There is considerable duplication here which can be confusing for new users at least. I wonder if we could improve this somehow.

I guess the tasks here are:

1. Decide on how to reduce the duplication and then improve the default formatter
2. Define a "consise formatter" which presents the same info with brevity

I'll start the discussion using the example above.

This provides me information about the type mismatch:

iex> AgeCheck.user_older_than?("foobar", 42)
** (TypeCheck.TypeError) At lib/type_check_example.ex:28:
The call to `user_older_than?/2` failed,
because parameter no. 1 does not adhere to the spec `%User{age: integer(), name: binary()}`.
Rather, its value is: `"foobar"`.

This tells me the same thing, with the following additional information:

• a spec of the function
• a more complete location of the function (eg. includes the app and module)
• that "foobar" is not a map

Details:
  The call `user_older_than?("foobar", 42)` 
  does not adhere to spec `user_older_than?(%User{age: integer(), name: binary()},  integer()) :: boolean()`. Reason:
    parameter no. 1:
      `"foobar"` does not check against `%User{age: integer(), name: binary()}`. Reason:
        `"foobar"` is not a map.
    (type_check_example 0.1.0) lib/type_check_example.ex:28: AgeCheck.user_older_than?/2

I don't know the default formatter well enough to suggest what's possible, but perhaps I can suggest something (slightly influenced by elm errors):

iex> AgeCheck.user_older_than?("foobar", 42)
** (TypeCheck.TypeError) At lib/type_check_example.ex:28:
The call to `user_older_than?/2` failed, because the 1st parameter does not adhere to the spec;

    user_older_than?(%User{age: integer(), name: binary()},  integer()) :: boolean()
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You have given the value:

                          "foobar"

But a value of the following type is required:

                          `%User{age: integer(), name: binary()}`

Details:
  reason: "is not a map"
  (type_check_example 0.1.0) lib/type_check_example.ex:28: AgeCheck.user_older_than?/2

I know the "details" section bounces down the types to get to the error, so it might be that this just doesn't work, but maybe this will start the discussion.