syscall: add DLLError.Unwrap function

Because we're expecting for future functions to be unavailable, we
should add an Unwrap() function to the DLLError struct, so that people
can test for this situation easily via:

    if errors.Is(err, syscall.ERROR_PROC_NOT_FOUND) { ... }

DLLError already was wrapping the underlying Errno error, but never got
the Go 1.13 helper method.

Fixes golang#42584

Change-Id: I0f32a5146946b1b37a30897ba825a56faefc792c
Reviewed-on: https://go-review.googlesource.com/c/go/+/269761
Run-TryBot: Jason A. Donenfeld <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Alex Brainman <[email protected]>
Trust: Alex Brainman <[email protected]>
Trust: Jason A. Donenfeld <[email protected]>

doc/go1.16.html

@@ -501,6 +501,10 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
     <p><!-- CL 261917 -->
       <a href="/pkg/syscall/#SysProcAttr"><code>SysProcAttr</code></a> on Windows has a new NoInheritHandles field that disables inheriting handles when creating a new process.
     </p>
+
+    <p><!-- CL 269761, golang.org/issue/42584 -->
+      <a href="/pkg/syscall/#DLLError"><code>DLLError</code></a> on Windows now has an Unwrap function for unwrapping its underlying error.
+    </p>
   </dd>
 </dl><!-- syscall -->
 

src/syscall/dll_windows.go

@@ -20,6 +20,8 @@ type DLLError struct {
 
 func (e *DLLError) Error() string { return e.Msg }
 
+func (e *DLLError) Unwrap() error { return e.Err }
+
 // Implemented in ../runtime/syscall_windows.go.
 
 func Syscall(trap, nargs, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)