Added return count mismatch error

src/errors/ReturnCountMismatch.go

@@ -0,0 +1,26 @@
+package errors
+
+import "fmt"
+
+// ReturnCountMismatch error is created when the number of returned values doesn't match the return type.
+type ReturnCountMismatch struct {
+	Count         int
+	ExpectedCount int
+}
+
+// Error generates the string representation.
+func (err *ReturnCountMismatch) Error() string {
+	values := "values"
+
+	if err.Count == 1 {
+		values = "value"
+	}
+
+	types := "types"
+
+	if err.ExpectedCount == 1 {
+		types = "type"
+	}
+
+	return fmt.Sprintf("Returns %d %s in a function with %d return %s", err.Count, values, err.ExpectedCount, types)
+}