stale ghc-toolchain target file will result in Prelude.read: no parse

Currently, hadrian reads the ghc-toolchain-or-./configure-generated .target files using read. This can give devs an awful Prelude.read: no parse error if they upgrade the compiler to a more recent version that extended the .target format but forget to reconfigure.

We should avoid read and instead provide a custom error...

Turns out this is compounded by hadrian caching dependencies on .target files and then reading the stale cached file rather than newly generated ones.