wire: fix error messages for Bind and InterfaceValue when the arg doesn't implement the interface (google/go-cloud#491)

Fixes google/go-cloud#490

internal/wire/testdata/InterfaceValueDoesntImplement/foo/foo.go

@@ -0,0 +1,21 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println(injectedMessage())
+}

internal/wire/testdata/InterfaceValueDoesntImplement/foo/wire.go

@@ -0,0 +1,28 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//+build wireinject
+
+package main
+
+import (
+	"github.com/google/go-cloud/wire"
+	"io"
+)
+
+func injectedMessage() string {
+	// wrong: string doesn't implement io.Reader.
+	wire.Build(wire.InterfaceValue(new(io.Reader), "bar"))
+	return ""
+}

internal/wire/testdata/InterfaceValueDoesntImplement/pkg

@@ -0,0 +1 @@
+example.com/foo

internal/wire/testdata/InterfaceValueDoesntImplement/want/wire_errs.txt

@@ -0,0 +1 @@
+string does not implement io.Reader