improve error when ".rackboot" section is stripped

Detect the case that a ".rackboot" ELF section is expected to be
present but isn't found, instead of failing with a more obscure
message about the format of the `petite` boot image.

racket/src/cs/c/main.c

@@ -109,12 +109,17 @@ static char *path_append_2(const char *p1, const char *p2) {
 #endif
 
 #if !defined(WIN32) && !defined(OS_X)
-static long find_boot_section(const char *me)
+static long find_boot_section(const char *me, int must_find)
 {
   int start = 0, end = 0;
 
   find_elf_section_offset(me, ".rackboot", &start, &end);
 
+  if (start == 0 && must_find) {
+    fprintf(stderr, "%s: ELF section \".rackboot\" is missing\n", me);
+    exit(1);
+  }
+
   return start;
 }
 #endif
@@ -202,7 +207,11 @@ static int bytes_main(int argc, char **argv,
   else
     boot_offset = 0;
 #else
-  boot_offset = find_boot_section(boot_exe);
+  boot_offset = find_boot_section(boot_exe,
+                                  /* If the first offset is 0 and the second is not,
+                                     then the intent must be for those offsets to
+                                     be relative to a boot section: */
+                                  (boot1_offset == 0) && (boot2_offset > 0));
 #endif
 
   boot1_offset += boot_offset;