Update vendor (#1265)

vendor/github.com/hashicorp/errwrap/README.md

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+# errwrap
+
+`errwrap` is a package for Go that formalizes the pattern of wrapping errors
+and checking if an error contains another error.
+
+There is a common pattern in Go of taking a returned `error` value and
+then wrapping it (such as with `fmt.Errorf`) before returning it. The problem
+with this pattern is that you completely lose the original `error` structure.
+
+Arguably the _correct_ approach is that you should make a custom structure
+implementing the `error` interface, and have the original error as a field
+on that structure, such [as this example](http://golang.org/pkg/os/#PathError).
+This is a good approach, but you have to know the entire chain of possible
+rewrapping that happens, when you might just care about one.
+
+`errwrap` formalizes this pattern (it doesn't matter what approach you use
+above) by giving a single interface for wrapping errors, checking if a specific
+error is wrapped, and extracting that error.
+
+## Installation and Docs
+
+Install using `go get github.com/hashicorp/errwrap`.
+
+Full documentation is available at
+http://godoc.org/github.com/hashicorp/errwrap
+
+## Usage
+
+#### Basic Usage
+
+Below is a very basic example of its usage:
+
+```go
+// A function that always returns an error, but wraps it, like a real
+// function might.
+func tryOpen() error {
+	_, err := os.Open("/i/dont/exist")
+	if err != nil {
+		return errwrap.Wrapf("Doesn't exist: {{err}}", err)
+	}
+
+	return nil
+}
+
+func main() {
+	err := tryOpen()
+
+	// We can use the Contains helpers to check if an error contains
+	// another error. It is safe to do this with a nil error, or with
+	// an error that doesn't even use the errwrap package.
+	if errwrap.Contains(err, "does not exist") {
+		// Do something
+	}
+	if errwrap.ContainsType(err, new(os.PathError)) {
+		// Do something
+	}
+
+	// Or we can use the associated `Get` functions to just extract
+	// a specific error. This would return nil if that specific error doesn't
+	// exist.
+	perr := errwrap.GetType(err, new(os.PathError))
+}
+```
+
+#### Custom Types
+
+If you're already making custom types that properly wrap errors, then
+you can get all the functionality of `errwraps.Contains` and such by
+implementing the `Wrapper` interface with just one function. Example:
+
+```go
+type AppError {
+  Code ErrorCode
+  Err  error
+}
+
+func (e *AppError) WrappedErrors() []error {
+  return []error{e.Err}
+}
+```
+
+Now this works:
+
+```go
+err := &AppError{Err: fmt.Errorf("an error")}
+if errwrap.ContainsType(err, fmt.Errorf("")) {
+	// This will work!
+}
+```