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/error.rs

@@ -1,42 +1,52 @@
-use error_chain::error_chain;
 use std::io::Write;
+use thiserror::Error;
 
-error_chain! {
-    foreign_links {
-        Clap(::clap::Error) #[cfg(feature = "minimal-application")];
-        Io(::std::io::Error);
-        SyntectError(::syntect::LoadingError);
-        ParseIntError(::std::num::ParseIntError);
-        GlobParsingError(::globset::Error);
-        SerdeYamlError(::serde_yaml::Error);
-    }
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error(transparent)]
+    Io(#[from] ::std::io::Error),
+    #[error(transparent)]
+    SyntectError(#[from] ::syntect::LoadingError),
+    #[error(transparent)]
+    ParseIntError(#[from] ::std::num::ParseIntError),
+    #[error(transparent)]
+    GlobParsingError(#[from] ::globset::Error),
+    #[error(transparent)]
+    SerdeYamlError(#[from] ::serde_yaml::Error),
+    #[error("unable to detect syntax for {0}")]
+    UndetectedSyntax(String),
+    #[error("unknown syntax: '{0}'")]
+    UnknownSyntax(String),
+    #[error("Unknown style '{0}'")]
+    UnknownStyle(String),
+    #[error("Use of bat as a pager is disallowed in order to avoid infinite recursion problems")]
+    InvalidPagerValueBat,
+    #[error("{0}")]
+    Msg(String),
+}
 
-    errors {
-        UndetectedSyntax(input: String) {
-            description("unable to detect syntax"),
-            display("unable to detect syntax for {}", input)
-        }
-        UnknownSyntax(name: String) {
-            description("unknown syntax"),
-            display("unknown syntax: '{}'", name)
-        }
-        InvalidPagerValueBat {
-            description("invalid value `bat` for pager property"),
-            display("Use of bat as a pager is disallowed in order to avoid infinite recursion problems")
-        }
+impl From<&'static str> for Error {
+    fn from(s: &'static str) -> Self {
+        Error::Msg(s.to_owned())
     }
 }
 
+impl From<String> for Error {
+    fn from(s: String) -> Self {
+        Error::Msg(s)
+    }
+}
+
+pub type Result<T> = std::result::Result<T, Error>;
+
 pub fn default_error_handler(error: &Error, output: &mut dyn Write) {
     use ansi_term::Colour::Red;
 
     match error {
-        Error(ErrorKind::Io(ref io_error), _)
-            if io_error.kind() == ::std::io::ErrorKind::BrokenPipe =>
-        {
+        Error::Io(ref io_error) if io_error.kind() == ::std::io::ErrorKind::BrokenPipe => {
             ::std::process::exit(0);
         }
-        Error(ErrorKind::SerdeYamlError(_), _) => {
+        Error::SerdeYamlError(_) => {
             writeln!(
                 output,
                 "{}: Error while parsing metadata.yaml file: {}",