Added error handling to LogfmtPrinter

Author: Bungeefan

lib/src/printers/logfmt_printer.dart

@@ -29,6 +29,9 @@ class LogfmtPrinter extends LogPrinter {
         }
       });
     }
+    if (event.error != null) {
+      output.write(' error="${event.error}"');
+    }
 
     return [output.toString()];
   }

test/printers/logfmt_printer_test.dart

@@ -1,5 +1,5 @@
-import 'package:test/test.dart';
 import 'package:logger/logger.dart';
+import 'package:test/test.dart';
 
 void main() {
   var printer = LogfmtPrinter();
@@ -18,13 +18,14 @@ void main() {
 
   test('with a string message includes a msg key', () {
     expect(
-        printer.log(LogEvent(
-          Level.debug,
-          'some message',
-          Exception('boom'),
-          StackTrace.current,
-        ))[0],
-        contains('msg="some message"'));
+      printer.log(LogEvent(
+        Level.debug,
+        'some message',
+        Exception('boom'),
+        StackTrace.current,
+      ))[0],
+      contains('msg="some message"'),
+    );
   });
 
   test('includes random key=value pairs', () {
@@ -39,6 +40,21 @@ void main() {
     expect(output, contains('foo="bar baz"'));
   });
 
+  test('handles an error/exception', () {
+    var output = printer.log(LogEvent(
+      Level.debug,
+      'some message',
+      Exception('boom'),
+      StackTrace.current,
+    ))[0];
+    expect(output, contains('error="Exception: boom"'));
+
+    output = printer.log(LogEvent(
+      Level.debug,
+      'some message',
+    ))[0];
+    expect(output, isNot(contains('error=')));
+  });
+
   test('handles a stacktrace', () {}, skip: 'TODO');
-  test('handles an error/exception', () {}, skip: 'TODO');
 }