Add Profiler Admin API (#6463)

Two handlers are added to admin API to enable profiling and disable
profiling of a server in a standalone mode, or all nodes in the
distributed mode.

/minio/admin/profiling/start/{cpu,block,mem}:
  - Start profiling and return starting JSON results, e.g. one
    node is offline.

/minio/admin/profiling/download:
  - Stop the on-going profiling task
  - Stream a zip file which contains all profiling files that can
    be later inspected by go tool pprof

cmd/api-errors.go

@@ -1727,7 +1727,10 @@ func toAPIErrorCode(err error) (apiErr APIErrorCode) {
 
 // getAPIError provides API Error for input API error code.
 func getAPIError(code APIErrorCode) APIError {
-	return errorCodeResponse[code]
+	if apiErr, ok := errorCodeResponse[code]; ok {
+		return apiErr
+	}
+	return errorCodeResponse[ErrInternalError]
 }
 
 // getErrorResponse gets in standard error and resource value and