Replace deprecated 'error-chain' with 'thiserror' (#1820)

We can't use #[from] on Error::Msg(String) because String does not implement Error.
(Which it shouldn't; see e.g. https://internals.rust-lang.org/t/impl-error-for-string/8881.)
So we implement From manually for Error::Msg, since our current code was written
in that way for error-chain.

.github/workflows/CICD.yml

@@ -37,7 +37,7 @@ jobs:
       uses: actions-rs/cargo@v1
       with:
         command: clippy
-        args: --locked --all-targets --all-features
+        args: --locked --all-targets --all-features -- --allow clippy::unknown_clippy_lints
     - name: Run tests
       uses: actions-rs/cargo@v1
       with: