Write error messages to pager, if attached

closes #946

src/error.rs

@@ -1,4 +1,5 @@
 use error_chain::error_chain;
+use std::io::Write;
 
 error_chain! {
     foreign_links {
@@ -11,7 +12,7 @@ error_chain! {
     }
 }
 
-pub fn default_error_handler(error: &Error) {
+pub fn default_error_handler(error: &Error, output: &mut dyn Write) {
     use ansi_term::Colour::Red;
 
     match error {
@@ -21,14 +22,16 @@ pub fn default_error_handler(error: &Error) {
             ::std::process::exit(0);
         }
         Error(ErrorKind::SerdeYamlError(_), _) => {
-            eprintln!(
+            writeln!(
+                output,
                 "{}: Error while parsing metadata.yaml file: {}",
                 Red.paint("[bat error]"),
                 error
-            );
+            )
+            .ok();
         }
         _ => {
-            eprintln!("{}: {}", Red.paint("[bat error]"), error);
+            writeln!(output, "{}: {}", Red.paint("[bat error]"), error).ok();
         }
     };
 }