feat: use %+v for displaying errors

Some error tracing libraries use this to write the full trace, which is
useful when debugging.

assert.go

@@ -148,7 +148,7 @@ func IsError(t testing.TB, err, target error, msgAndArgs ...any) {
 		return
 	}
 	t.Helper()
-	t.Fatal(formatMsgAndArgs(fmt.Sprintf("Error tree %q should contain error %q", err, target), msgAndArgs...))
+	t.Fatal(formatMsgAndArgs(fmt.Sprintf("Error tree %+v should contain error %q", err, target), msgAndArgs...))
 }
 
 // NotIsError asserts than no error in "err"'s tree matches "target".
@@ -157,7 +157,7 @@ func NotIsError(t testing.TB, err, target error, msgAndArgs ...any) {
 		return
 	}
 	t.Helper()
-	t.Fatal(formatMsgAndArgs(fmt.Sprintf("Error tree %q should NOT contain error %q", err, target), msgAndArgs...))
+	t.Fatal(formatMsgAndArgs(fmt.Sprintf("Error tree %+v should NOT contain error %q", err, target), msgAndArgs...))
 }
 
 // Error asserts that an error is not nil.
@@ -176,7 +176,7 @@ func NoError(t testing.TB, err error, msgAndArgs ...any) {
 	}
 	t.Helper()
 	msg := formatMsgAndArgs("Did not expect an error but got:", msgAndArgs...)
-	t.Fatalf("%s\n%s", msg, err)
+	t.Fatalf("%s\n%+v", msg, err)
 }
 
 // True asserts that an expression is true.

assert_test.go

@@ -86,6 +86,24 @@ func TestEqualError(t *testing.T) {
 	})
 }
 
+func TestError(t *testing.T) {
+	assertOk(t, "Error", func(t testing.TB) {
+		Error(t, fmt.Errorf("hello"))
+	})
+	assertFail(t, "Nil", func(t testing.TB) {
+		Error(t, nil)
+	})
+}
+
+func TestNoError(t *testing.T) {
+	assertOk(t, "Nil", func(t testing.TB) {
+		NoError(t, nil)
+	})
+	assertFail(t, "Error", func(t testing.TB) {
+		NoError(t, fmt.Errorf("hello"))
+	})
+}
+
 func TestZero(t *testing.T) {
 	assertOk(t, "Struct", func(t testing.TB) {
 		Zero(t, Data{})