Impl and tests for fluent.Recover.

Used the ipldfree.Node impl concretely to make the tests work.
In theory any impl should of course work, but it's not useful or
interesting to do a compatibility matrix here -- particularly given
that we use the fluent system in the compataibility tests for all
the other Node impl packages.
Signed-off-by: Eric Myhre <hash@exultant.us>

fluent/fluentRecover.go

 package fluent
 
-import "fmt"
-
-func Recover(fn func()) error {
-	return fmt.Errorf("TODO")
-	// recover, but only handle fluent.Error
-	// re-raise anything else
+// Recover invokes a function within a panic-recovering context, and returns
+// any raised fluent.Error values; any other values are re-panicked.
+//
+// This can be useful for writing large blocks of code using fluent nodes,
+// and handling any errors at once at the end.
+func Recover(fn func()) (err error) {
+	defer func() {
+		ei := recover()
+		switch e2 := ei.(type) {
+		case nil:
+			return
+		case Error:
+			err = e2
+		default:
+			panic(ei)
+		}
+	}()
+	fn()
+	return
 }

fluent/fluentRecover_test.go

+package fluent
+
+import (
+	"fmt"
+	"testing"
+
+	. "github.com/warpfork/go-wish"
+
+	"github.com/ipld/go-ipld-prime/impl/free"
+)
+
+func TestRecover(t *testing.T) {
+	t.Run("simple traversal error should capture", func(t *testing.T) {
+		Wish(t,
+			Recover(func() {
+				WrapNode(&ipldfree.Node{}).TraverseIndex(0).AsString()
+				t.Fatal("should not be reached")
+			}),
+			ShouldEqual,
+			Error{fmt.Errorf("cannot traverse a node that is undefined")},
+		)
+	})
+	t.Run("correct traversal should return nil", func(t *testing.T) {
+		Wish(t,
+			Recover(func() {
+				n := &ipldfree.Node{}
+				n.SetString("foo")
+				WrapNode(n).AsString()
+			}),
+			ShouldEqual,
+			nil,
+		)
+	})
+	t.Run("other panics should continue to rise", func(t *testing.T) {
+		Wish(t,
+			func() (r interface{}) {
+				defer func() { r = recover() }()
+				Recover(func() {
+					panic("fuqawds")
+				})
+				return
+			}(),
+			ShouldEqual,
+			"fuqawds",
+		)
+	})
+}