3.1.3-RC2 is not able to compile `scala3-compiler` from current `main`

[Kordyjan]

Compiler version

3.1.3-RC2

Output

[error] -- [E007] Type Mismatch Error: /Users/pmarks/dotty/compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala:126:18
[error] 126 |              |  $ex
[error]     |                  ^^
[error]     |                  Found:    (ex : Throwable)
[error]     |                  Required: dotty.tools.dotc.printing.Formatting.Shown
[error]     |
[error]     | longer explanation available when compiling with `-explain`
[error] one error found
[error] (scala3-compiler / Compile / compileIncremental) Compilation failed

Expectation

Stuff compiles!

[nicolasstucki]

Probably introduced in #14455

[nicolasstucki]

We seem to be missing a given Show[Throwable] but we have one for Show[Exception].

https://github.com/lampepfl/dotty/pull/14455/files#diff-1aeccc1856f9265f1358bb106fe44428b9aa9b4d353bd1a85b5799a00e810b9cR69