refactor(loader): Throw when template not found

Sources/Errors.swift

@@ -0,0 +1,14 @@
+public class TemplateDoesNotExist: Error, CustomStringConvertible {
+  let templateNames: [String]
+  let loader: Loader
+
+  public init(templateNames: [String], loader: Loader) {
+    self.templateNames = templateNames
+    self.loader = loader
+  }
+
+  public var description: String {
+    let templates = templateNames.joined(separator: ", ")
+    return "Template named `\(templates)` does not exist in loader \(loader)"
+  }
+}