all: replace more panic(wire.Build(...)) calls with wire.Build(...); return stuff (google/go-cloud#248)

Fixes google/go-cloud#246

internal/wire/testdata/MultipleMissingInputs/foo/wire.go

@@ -21,5 +21,6 @@ import (
 )
 
 func injectBaz() Baz {
-	panic(wire.Build(provideBaz))
+	wire.Build(provideBaz)
+	return Baz(0)
 }

wire.go

@@ -30,7 +30,12 @@ func NewSet(...interface{}) ProviderSet {
 // providers to use. Its arguments are the same as NewSet. Its return
 // value is an error message that can be sent to panic.
 //
-// Example:
+// Examples:
+//
+//	func injector(ctx context.Context) (*sql.DB, error) {
+//		wire.Build(otherpkg.FooSet, myProviderFunc)
+//		return nil, nil
+//	}
 //
 //	func injector(ctx context.Context) (*sql.DB, error) {
 //		panic(wire.Build(otherpkg.FooSet, myProviderFunc))