return 404 code when delete not exist experiment

[kadern0]

Fixes #93

 curl -X DELETE 127.0.0.1:31767/api/attack/1234 -w "%{http_code}" 
{"error":true,"message":"error.api.resource_not_found: record not found","code":"error.api.resource_not_found","full_text":"error.api.resource_not_found: record not found\n at github.com/chaos-mesh/chaosd/pkg/server/httpserver.handleError()\n\t/home/pcaderno/go/src/github.com/kadern0/chaosd/pkg/server/httpserver/server.go:277\n at github.com/chaos-mesh/chaosd/pkg/server/httpserver.(*httpServer).recoverAttack()\n\t/home/pcaderno/go/src/github.com/kadern0/chaosd/pkg/server/httpserver/server.go:268\n at github.com/gin-gonic/gin.(*Context).Next()\n\t/home/pcaderno/go/pkg/mod/github.com/gin-gonic/gin@v1.6.3/context.go:161\n at github.com/chaos-mesh/chaosd/pkg/server/utils.MWHandleErrors.func1()\n\t/home/pcaderno/go/src/github.com/kadern0/chaosd/pkg/server/utils/error.go:47\n at github.com/gin-gonic/gin.(*Context).Next()\n\t/home/pcaderno/go/pkg/mod/github.com/gin-gonic/gin@v1.6.3/context.go:161\n at github.com/gin-gonic/gin.RecoveryWithWriter.func1()\n\t/home/pcaderno/go/pkg/mod/github.com/gin-gonic/gin@v1.6.3/recovery.go:83\n at github.com/gin-gonic/gin.(*Context).Next()\n\t/home/pcaderno/go/pkg/mod/github.com/gin-gonic/gin@v1.6.3/context.go:161\n at github.com/gin-gonic/gin.LoggerWithConfig.func1()\n\t/home/pcaderno/go/pkg/mod/github.com/gin-gonic/gin@v1.6.3/logger.go:241\n at github.com/gin-gonic/gin.(*Context).Next()\n\t/home/pcaderno/go/pkg/mod/github.com/gin-gonic/gin@v1.6.3/context.go:161\n at github.com/gin-gonic/gin.(*Engine).handleHTTPRequest()\n\t/home/pcaderno/go/pkg/mod/github.com/gin-gonic/gin@v1.6.3/gin.go:409\n at github.com/gin-gonic/gin.(*Engine).ServeHTTP()\n\t/home/pcaderno/go/pkg/mod/github.com/gin-gonic/gin@v1.6.3/gin.go:367\n at net/http.serverHandler.ServeHTTP()\n\t/usr/local/go/src/net/http/server.go:2887\n at net/http.(*conn).serve()\n\t/usr/local/go/src/net/http/server.go:1952\n at runtime.goexit()\n\t/usr/local/go/src/runtime/asm_amd64.s:1371"}404

[WangXiangUSTC]

Thanks！ LGTM

[Andrewmatilde]

LGTM