wire: don't fail at first error (google/go-cloud#204)

This changes both Generate and Load to try to advance as far as possible
while collecting errors. A small helper type, errorCollector, is used to
make control flow easier to read.

Fixes google/go-cloud#5

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

@@ -0,0 +1,29 @@
+// Copyright 2018 Google LLC
+//
+// 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(injectBaz())
+}
+
+type Foo int
+type Bar int
+type Baz int
+
+func provideBaz(foo Foo, bar Bar) Baz {
+	return 0
+}

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

@@ -0,0 +1,25 @@
+// Copyright 2018 Google LLC
+//
+// 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"
+)
+
+func injectBaz() Baz {
+	panic(wire.Build(provideBaz))
+}

internal/wire/testdata/MultipleMissingInputs/out.txt

@@ -0,0 +1,3 @@
+ERROR
+no provider found for foo.Foo
+no provider found for foo.Bar

internal/wire/testdata/MultipleMissingInputs/pkg

@@ -0,0 +1 @@
+foo