Manually implement Display/Error for Error

[coolreader18]

This improves build times for crates that depend on this crate, since instead of making a chain in the build graph like this:

• foo-macro
  • syn
  • proc-macro-crate
    • thiserror
      • syn

Which forces syn to be built in series (syn then proc-macro-crate then foo-macro), it instead can look like

• foo-macro
  • syn
  • proc-macro-crate

Which lets it build in parallel (syn AND proc-macro-crate, then foo-macro).