all: add Unwrap and Is methods to various error types

Add Unwrap methods to types which wrap an underlying error:

  "encodinc/csv".ParseError
  "encoding/json".MarshalerError
  "net/http".transportReadFromServerError
  "net".OpError
  "net".DNSConfigError
  "net/url".Error
  "os/exec".Error
  "signal/internal/pty".PtyError
  "text/template".ExecError

Add os.ErrTemporary. A case could be made for putting this error
value in package net, since no exported error types in package os
include a Temporary method. However, syscall errors returned from
the os package do include this method.

Add Is methods to error types with a Timeout or Temporary method,
making errors.Is(err, os.Err{Timeout,Temporary}) equivalent to
testing the corresponding method:

  "context".DeadlineExceeded
  "internal/poll".TimeoutError
  "net".adrinfoErrno
  "net".OpError
  "net".DNSError
  "net/http".httpError
  "net/http".tlsHandshakeTimeoutError
  "net/pipe".timeoutError
  "net/url".Error

Updates #30322
Updates #29934

Change-Id: I409fb20c072ea39116ebfb8c7534d493483870dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/170037
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/src/os/exec/exec.go b/src/os/exec/exec.go
index 9a9265b667..17ef003eca 100644
--- a/src/os/exec/exec.go
+++ b/src/os/exec/exec.go
@@ -47,6 +47,8 @@ func (e *Error) Error() string {
 	return "exec: " + strconv.Quote(e.Name) + ": " + e.Err.Error()
 }
 
+func (e *Error) Unwrap() error { return e.Err }
+
 // Cmd represents an external command being prepared or run.
 //
 // A Cmd cannot be reused after calling its Run, Output or CombinedOutput