Add errgroup to image-slender

[mi-bear]

ʕ•ᴥ•ʔがどんどん痩せます。
sync.ErrGroup を利用した Go の並行処理に変更しています。

master:

ʕ'ᴥ'ʔ❯❯❯ go test -count 10 -test.bench BenchmarkExecute
goos: darwin
goarch: amd64
pkg: github.com/WomenWhoGoTokyo/codelab/image-slender
BenchmarkExecute-8   	       1	15734196765 ns/op
BenchmarkExecute-8   	       1	15749080551 ns/op
BenchmarkExecute-8   	       1	15740172433 ns/op
BenchmarkExecute-8   	       1	15733120724 ns/op
BenchmarkExecute-8   	       1	15711674788 ns/op
BenchmarkExecute-8   	       1	15712443844 ns/op
BenchmarkExecute-8   	       1	15691785071 ns/op
BenchmarkExecute-8   	       1	15713679347 ns/op
BenchmarkExecute-8   	       1	15723890234 ns/op
BenchmarkExecute-8   	       1	15706823340 ns/op
PASS
ok  	github.com/WomenWhoGoTokyo/codelab/image-slender	157.238s

add-codelab-contents-image-slender:

ʕ'ᴥ'ʔ❯❯❯ go test -count 10 -test.bench BenchmarkExecute

goos: darwin
goarch: amd64
pkg: github.com/WomenWhoGoTokyo/codelab/image-slender
BenchmarkExecute-8   	       1	3395237824 ns/op
BenchmarkExecute-8   	       1	3356718806 ns/op
BenchmarkExecute-8   	       1	3364689756 ns/op
BenchmarkExecute-8   	       1	3340897141 ns/op
BenchmarkExecute-8   	       1	3353635043 ns/op
BenchmarkExecute-8   	       1	3361589889 ns/op
BenchmarkExecute-8   	       1	3362286656 ns/op
BenchmarkExecute-8   	       1	3340086297 ns/op
BenchmarkExecute-8   	       1	3358628977 ns/op
BenchmarkExecute-8   	       1	3351012431 ns/op
PASS
ok  	github.com/WomenWhoGoTokyo/codelab/image-slender	33.618s