[check-iwyu] Use echoerr where sensible

tools/check-iwyu

@@ -44,7 +44,7 @@ if [[ ! -v IWYU_TOOL ]]; then
 	done
 
 	if [[ ! -v IWYU_TOOL ]]; then
-		echo "iwyu_tool not found"
+		echoerr "iwyu_tool not found"
 		exit 1
 	fi
 fi
@@ -89,7 +89,7 @@ ERROR_STRINGS+=("fatal error:")
 
 for ERROR_STRING in "${ERROR_STRINGS[@]}"; do
 	if grep -q "${ERROR_STRING}" "${IWYU_LOG}"; then
-		echo "IWYU found errors!"
+		echoerr "IWYU found errors!"
 		cat "${IWYU_LOG}"
 		exit 1
 	fi