feature: add foldCause to HttpError (#1066)

zio-http/src/main/scala/zhttp/http/HttpError.scala

@@ -1,6 +1,18 @@
 package zhttp.http
 
+import zhttp.http.HttpError.HTTPErrorWithCause
+
 sealed abstract class HttpError(val status: Status, val message: String) extends Throwable(message) { self =>
+  def foldCause[A](a: A)(f: Throwable => A): A = self match {
+    case error: HTTPErrorWithCause =>
+      error.cause match {
+        case Some(throwable) => f(throwable)
+        case None            => a
+      }
+    case _                         => a
+
+  }
+
   def toResponse: Response = Response.fromHttpError(self)
 }
 

zio-http/src/test/scala/zhttp/http/HttpErrorSpec.scala

@@ -0,0 +1,21 @@
+package zhttp.http
+
+import zio.test.Assertion.equalTo
+import zio.test.{DefaultRunnableSpec, assert}
+
+object HttpErrorSpec extends DefaultRunnableSpec {
+  def spec = suite("HttpError") {
+    suite("foldCause") {
+      test("should fold the cause") {
+        val error  = HttpError.InternalServerError(cause = Option(new Error("Internal server error")))
+        val result = error.foldCause("")(cause => cause.getMessage)
+        assert(result)(equalTo("Internal server error"))
+      } +
+        test("should fold with no cause") {
+          val error  = HttpError.NotFound(!!)
+          val result = error.foldCause("Page not found")(cause => cause.getMessage)
+          assert(result)(equalTo("Page not found"))
+        }
+    }
+  }
+}