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.

src/printer.rs

@@ -174,7 +174,7 @@ impl<'a> InteractivePrinter<'a> {
             let syntax_in_set =
                 match assets.get_syntax(config.language, input, &config.syntax_mapping) {
                     Ok(syntax_in_set) => syntax_in_set,
-                    Err(Error(ErrorKind::UndetectedSyntax(_), _)) => {
+                    Err(Error::UndetectedSyntax(_)) => {
                         let syntax_set = assets.get_syntax_set()?;
                         let syntax = syntax_set.find_syntax_plain_text();
                         SyntaxReferenceInSet { syntax, syntax_set }