[tools] improve check-iwyu script

tools/check-iwyu

@@ -96,12 +96,13 @@ fi
 # See
 # - https://github.com/include-what-you-use/include-what-you-use/issues/789
 # - https://github.com/include-what-you-use/include-what-you-use/commit/a7499e4a2b416592777cc4c33fca746d091af738
-ERROR_STRINGS=()
-ERROR_STRINGS+=("should add these lines:")
-ERROR_STRINGS+=("fatal error:")
+ERROR_STRINGS=(
+	"should add these lines:"
+	"should remove these lines:"
+)
 
 for ERROR_STRING in "${ERROR_STRINGS[@]}"; do
-	if grep -q "${ERROR_STRING}" "${IWYU_LOG}"; then
+	if grep -qF "${ERROR_STRING}" "${IWYU_LOG}"; then
 		echoerr "IWYU found errors!"
 		cat "${IWYU_LOG}"
 		exit 1