Enhance the ExUnit.AssertionError message.

[ericentin]

I would like to pad the message better, at the moment it ends up looking like this:

iex(2)> assert :a == :b
** (ExUnit.AssertionError) Assertion with == failed
code: :a == :b
lhs:  :a
rhs:  :b

    lib/ex_unit/assertions.ex:287: ExUnit.Assertions.assert/2

iex(2)> spawn_link fn -> assert :a == :b end
** (EXIT from #PID<0.57.0>) an exception was raised:
    ** (ExUnit.AssertionError) Assertion with == failed
code: :a == :b
lhs:  :a
rhs:  :b

        lib/ex_unit/assertions.ex:287: ExUnit.Assertions.assert/2

Since we don't have any information about how nested the messages are in Exception.message, adding the appropriate padding seems very difficult, if not impossible. Happy to accept any feedback on how we should format it, given that limitation. I was considering adding an additional newline between the note and code like this:

** (ExUnit.AssertionError) 
Assertion with == failed
code: :a == :b
lhs:  :a
rhs:  :b

    lib/ex_unit/assertions.ex:287: ExUnit.Assertions.assert/2

iex(2)> spawn_link fn -> assert :a == :b end
** (EXIT from #PID<0.57.0>) an exception was raised:
    ** (ExUnit.AssertionError) 
Assertion with == failed
code: :a == :b
lhs:  :a
rhs:  :b

        lib/ex_unit/assertions.ex:287: ExUnit.Assertions.assert/2

but I'm not sure if that makes it look better or worse. :)

\cc @josevalim

[josevalim]

A new line works nicely for me. :)

[josevalim]

If you add a new line though, won't it change how all errors are displayed by ExUnit CLI formatter?

[josevalim]

Let's change :colors_enabled? to :diff_enabled? and make the default be false. This way we won't need to check for it here: https://github.com/elixir-lang/elixir/pull/4709/files#diff-699b0020c6a196231a7d473322f1744dR24

We probably use colors_enabled? in a couple other places, so let's not forget to change those. :)

[ericentin]

@josevalim not if I only add it in ExUnit.AssertionError.message/1!

[ericentin]

Ok, looks like this now, which is pretty nice:

iex(2)> assert :a == :b
** (ExUnit.AssertionError)

Assertion with == failed
code: :a == :b
lhs:  :a
rhs:  :b

    lib/ex_unit/assertions.ex:288: ExUnit.Assertions.assert/2

iex(2)> spawn_link fn -> assert :a = :b end
** (EXIT from #PID<0.57.0>) an exception was raised:
    ** (ExUnit.AssertionError)

match (=) failed
code: :a = :b
rhs:  :b

        (stdlib) erl_eval.erl:670: :erl_eval.do_apply/6
        (stdlib) erl_eval.erl:438: :erl_eval.expr/5
        (stdlib) erl_eval.erl:122: :erl_eval.exprs/5

[josevalim]

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