grep.inc fixes

Wrong comment and unclear error message leads to usage troubles.

mysql-test/include/grep.inc

 # Grep file for regular expression and output to STDOUT
 #
 # Usage:
-# --let $grep_file= /path/to/your/file
-# --let $grep_regex= your_regex_string
+# --let grep_file= /path/to/your/file
+# --let grep_regex= your_regex_string
+# --let grep_once= your_regex_string_to_match_once
 # --source include/grep.inc
 
 --perl
-  open (my $fh, "<", "$ENV{grep_file}") or die $!;
+  die "grep_file not set!\n" unless $ENV{grep_file};
+  open (my $fh, "<", "$ENV{grep_file}") or die "$ENV{grep_file}: $!\n";
   while (<$fh>)
   {
-    /$ENV{grep_regex}/ &&
+    if (/$ENV{grep_regex}/) {
       print;
+    } elsif (defined $ENV{grep_once} && /$ENV{grep_once}/) {
+      print;
+      last;
+    }
   }
   close $fh;
 EOF