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.
As mentioned on #1334 `bat` should not be used as a value for `pager`,
this change checks both the balue of `bat` provided as a parameter or
as an environment variable.
