Improve error message when reading invalid `.target` files

A `.target` file generated by ghc-toolchain or by configure can become
invalid if the target representation (`Toolchain.Target`) is changed
while the files are not re-generated by calling `./configure` or
`ghc-toolchain` again. There is also the issue of hadrian caching the
dependencies on `.target` files, which makes parsing fail when reading
reading the cached value if the representation has been updated.

This patch provides a better error message in both situations, moving
away from a terrible `Prelude.read: no parse` error that you would get
otherwise.

Fixes #24199