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.

Cargo.toml

@@ -48,6 +48,7 @@ ansi_colours = "^1.0"
 console = "0.14.1"
 lazy_static = { version = "1.4", optional = true }
 lazycell = "1.0"
+thiserror = "1.0"
 wild = { version = "2.0", optional = true }
 content_inspector = "0.2.4"
 encoding = "0.2"
@@ -79,10 +80,6 @@ optional = true
 default-features = false
 features = ["suggestions", "color", "wrap_help"]
 
-[dependencies.error-chain]
-version = "0.12"
-default-features = false
-
 [dev-dependencies]
 assert_cmd = "1.0.8"
 serial_test = "0.5.1"