Avoid hard-coding a redundant "FATAL:" in RGBLINK

test/link/overlay/unfixed/a.asm

@@ -0,0 +1,4 @@
+FOR n, 15
+	SECTION "test {d:n}", ROM0
+	db n
+ENDR

test/link/overlay/unfixed/out.err

@@ -0,0 +1,13 @@
+FATAL: All sections must be fixed when using an overlay file; 15 are not:
+- "test 14"
+- "test 13"
+- "test 12"
+- "test 11"
+- "test 10"
+- "test 9"
+- "test 8"
+- "test 7"
+- "test 6"
+- "test 5"
+- and 5 more
+Linking aborted with 1 error

test/link/test.sh

@@ -233,6 +233,14 @@ tryDiff "$test"/out.err "$outtemp"
 tryCmp "$test"/out.gb "$gbtemp"
 evaluateTest
 
+test="overlay/unfixed"
+startTest
+"$RGBASM" -o "$otemp" "$test"/a.asm
+continueTest
+rgblinkQuiet -o "$gbtemp" -O "$test"/overlay.gb "$otemp" 2>"$outtemp"
+tryDiff "$test"/out.err "$outtemp"
+evaluateTest
+
 test="overlay/tiny"
 startTest
 "$RGBASM" -o "$otemp" "$test"/a.asm