Set set +e in generate.sh (go generate) to output correct information

While playing with go in tensorflow, the `go generate` will
not output correct information about missing protoc even though
generate.sh contains the following:
```
  PATH_PROTOC=$(which protoc)
  if [ ! -x "${PATH_PROTOC}" ]
  then
    echo "Protocol buffer compiler protoc not found in PATH or in ${PROTOC}"
    echo "Perhaps build it using:"
    echo "bazel build --config opt @protobuf//:protoc"
    exit 1
  fi
  PROTOC=$PATH_PROTOC
```

Instread, a non-informative error is displayed:
```
root@99829f070f46:/go/src/github.com/tensorflow/tensorflow# go generate github.com/tensorflow/tensorflow/tensorflow/go/op
../genop/main.go:15: running "sh": exit status 1
tensorflow/go/op/generate.go:15: running "go": exit status 1
root@99829f070f46:/go/src/github.com/tensorflow/tensorflow#
```

The reason is that `set -e` is at the beginning of the script and it exit
immediately at `PATH_PROTOC=$(which protoc)`.

This fix sets `set +e` before `PATH_PROTOC=$(which protoc)` and restores `set -e`
back, so that information about missing protoc outputed.

Below is the new output:
```
root@99829f070f46:/go/src/github.com/tensorflow/tensorflow# go generate github.com/tensorflow/tensorflow/tensorflow/go/op
Protocol buffer compiler protoc not found in PATH or in /go/src/github.com/tensorflow/tensorflow/bazel-out/host/bin/external/protobuf/protoc
Perhaps build it using:
bazel build --config opt @protobuf//:protoc
../genop/main.go:15: running "sh": exit status 1
tensorflow/go/op/generate.go:15: running "go": exit status 1
root@99829f070f46:/go/src/github.com/tensorflow/tensorflow#
```

Signed-off-by: Yong Tang <[email protected]>

Author: yongtang

tensorflow/go/genop/generate.sh

@@ -25,6 +25,7 @@ PROTOC="${TF_DIR}/bazel-out/host/bin/external/protobuf/protoc"
 
 if [ ! -x "${PROTOC}" ]
 then
+  set +e
   PATH_PROTOC=$(which protoc)
   if [ ! -x "${PATH_PROTOC}" ]
   then
@@ -34,6 +35,7 @@ then
     exit 1
   fi
   PROTOC=$PATH_PROTOC
+  set -e
 fi
 
 # Ensure that protoc-gen-go is available in $PATH