extra: error message should reflact that RUST_TEST_TASKS should be strictly positive (zero is illegal).

src/libextra/test.rs

@@ -745,7 +745,7 @@ fn get_concurrency() -> uint {
             let opt_n: Option<uint> = FromStr::from_str(s);
             match opt_n {
                 Some(n) if n > 0 => n,
-                _ => fail!("RUST_TEST_TASKS is `%s`, should be a non-negative integer.", s)
+                _ => fail!("RUST_TEST_TASKS is `%s`, should be a positive integer.", s)
             }
         }
         None => {

src/test/run-fail/test-tasks-invalid-value.rs

@@ -11,7 +11,7 @@
 // This checks that RUST_TEST_TASKS not being 1, 2, ... is detected
 // properly.
 
-// error-pattern:should be a non-negative integer
+// error-pattern:should be a positive integer
 // compile-flags: --test
 // exec-env:RUST_TEST_TASKS=foo