Send progress to stderr (#394)

By default, progress is sent to stdout, which is buffered and cannot be proerly processed by `quickgui`

quickget

@@ -545,7 +545,7 @@ function web_get() {
     fi
 
     if command -v aria2c &>/dev/null; then
-        if ! aria2c -x16 --continue=true --summary-interval=0 --download-result=hide --console-log-level=error "${URL}" -o "${DIR}/${FILE}"; then
+        if ! aria2c --stderr -x16 --continue=true --summary-interval=0 --download-result=hide --console-log-level=error "${URL}" -o "${DIR}/${FILE}"; then
           echo #Necessary as aria2c in suppressed mode does not have new lines
           echo "ERROR! Failed to download ${URL} with aria2c. Try running 'quickget' again."
           exit 1