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 (closed)