Better error messages

[adamw]

This is probably a large issue, with multiple improvements that can be done.

For example, when removing the given Print[Int] = _.toString from the readme example, the error message is:

[error] -- [E008] Not Found Error: Test.scala:33:77 
[error] 33 |  println(Tree.Branch(Tree.Branch(Tree.Leaf(1), Tree.Leaf(2)), Tree.Leaf(3)).print)
[error]    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]    |value print is not a member of Test.Tree.Branch[Int].
[error]    |An extension method was tried, but could not be fully constructed:
[error]    |
[error]    |    Test.Tree.derived$Print[T](
[error]    |      Test.Print.derived[A](
[error]    |        /* missing */
[error]    |          summon[
[error]    |            deriving.Mirror{
[error]    |              MirroredType >: Int; MirroredMonoType >: Int; 
[error]    |                MirroredElemTypes <: Tuple
[error]    |            }
[error]    |          ]
[error]    |      )
[error]    |    ).print()
[error] one error found

Maybe we could do better here?

[adamw]

Currently derivations often fail with messages like:

cannot reduce summonFrom with (...)

TODO: example

[adamw]

Promised example, using the classes from tests:

case class Param(a: String, b: String)
case class TestEntry(param: Param)

When you do:

SemiPrint.derived[TestEntry].print(TestEntry(Param("a", "b")))

you get:

[error] -- Error: /Users/adamw/projects/magnolia/src/test/tests.scala:234:14 -----------
[error] 234 |    val res = SemiPrint.derived[TestEntry].print(TestEntry(Param("a", "b")))
[error]     |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]     |cannot reduce summonFrom with
[error]     | patterns :  case given t @ _:magnolia.examples.SemiPrint[magnolia.tests.Param]
[error]     | This location contains code that was inlined from package.scala:140
[error]     | This location contains code that was inlined from magnolia.scala:52
[error]     | This location contains code that was inlined from magnolia.scala:13
[error]     | This location contains code that was inlined from magnolia.scala:88
[error]     | This location contains code that was inlined from magnolia.scala:90
[error] one error found

Which could be better :)