diversify "unable to spawn" failure messages

to help understand where a spurious failure is occurring

src/main.zig

@@ -5244,7 +5244,7 @@ fn cmdBuild(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
                 std.debug.lockStdErr();
                 defer std.debug.unlockStdErr();
                 break :t child.spawnAndWait() catch |err| {
-                    fatal("unable to spawn {s}: {s}", .{ child_argv.items[0], @errorName(err) });
+                    fatal("failed to spawn build runner {s}: {s}", .{ child_argv.items[0], @errorName(err) });
                 };
             };