fluent/qp: don't panic on string panics

Since qp uses panics as an error mechanism, the top-level functions
recover those errors to return them normally.

However, this blows up if, internally, an ipld.Node implementation
panics with a value whose type is not error. For example, with:

	panic("some panic message")

One gets:

	panic: some panic message [recovered]
		panic: interface conversion: string is not error: missing method Error

Fix this by having a fallback for non-error panic values.

fluent/qp/qp.go

@@ -2,6 +2,8 @@
 package qp
 
 import (
+	"fmt"
+
 	"github.com/ipld/go-ipld-prime"
 )
 
@@ -10,7 +12,12 @@ type Assemble = func(ipld.NodeAssembler)
 func BuildMap(np ipld.NodePrototype, sizeHint int64, fn func(ipld.MapAssembler)) (_ ipld.Node, err error) {
 	defer func() {
 		if r := recover(); r != nil {
-			err = r.(error)
+			if rerr, ok := r.(error); ok {
+				err = rerr
+			} else {
+				// A reasonable fallback, for e.g. strings.
+				err = fmt.Errorf("%v", r)
+			}
 		}
 	}()
 	nb := np.NewBuilder()
@@ -49,7 +56,12 @@ func MapEntry(ma ipld.MapAssembler, k string, fn Assemble) {
 func BuildList(np ipld.NodePrototype, sizeHint int64, fn func(ipld.ListAssembler)) (_ ipld.Node, err error) {
 	defer func() {
 		if r := recover(); r != nil {
-			err = r.(error)
+			if rerr, ok := r.(error); ok {
+				err = rerr
+			} else {
+				// A reasonable fallback, for e.g. strings.
+				err = fmt.Errorf("%v", r)
+			}
 		}
 	}()
 	nb := np.NewBuilder()