SugaredLogger/sweetenFields: Don't panic (#949)

SugaredLogger should not cause panics in user code.

For keys without matching values or non-string keys, instead of
panicking, log an error and move on.

Resolves #896

sugar.go

@@ -266,7 +266,7 @@ func (s *SugaredLogger) sweetenFields(args []interface{}) []Field {
 
 		// Make sure this element isn't a dangling key.
 		if i == len(args)-1 {
-			s.base.DPanic(_oddNumberErrMsg, Any("ignored", args[i]))
+			s.base.Error(_oddNumberErrMsg, Any("ignored", args[i]))
 			break
 		}
 
@@ -287,7 +287,7 @@ func (s *SugaredLogger) sweetenFields(args []interface{}) []Field {
 
 	// If we encountered any invalid key-value pairs, log an error.
 	if len(invalid) > 0 {
-		s.base.DPanic(_nonStringKeyErrMsg, Array("invalid", invalid))
+		s.base.Error(_nonStringKeyErrMsg, Array("invalid", invalid))
 	}
 	return fields
 }

sugar_test.go

@@ -36,13 +36,13 @@ func TestSugarWith(t *testing.T) {
 	// Convenience functions to create expected error logs.
 	ignored := func(msg interface{}) observer.LoggedEntry {
 		return observer.LoggedEntry{
-			Entry:   zapcore.Entry{Level: DPanicLevel, Message: _oddNumberErrMsg},
+			Entry:   zapcore.Entry{Level: ErrorLevel, Message: _oddNumberErrMsg},
 			Context: []Field{Any("ignored", msg)},
 		}
 	}
 	nonString := func(pairs ...invalidPair) observer.LoggedEntry {
 		return observer.LoggedEntry{
-			Entry:   zapcore.Entry{Level: DPanicLevel, Message: _nonStringKeyErrMsg},
+			Entry:   zapcore.Entry{Level: ErrorLevel, Message: _nonStringKeyErrMsg},
 			Context: []Field{Array("invalid", invalidPairs(pairs))},
 		}
 	}