Use more semantic exit codes so we can detect HTTP errors.

utils/pub/exit_codes.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// Exit code constants. From [the BSD sysexits manpage][manpage]. Not every
+/// constant here is used, even though some of the unused ones may be
+/// appropriate for errors encountered by pub.
+///
+/// [manpage]: http://www.freebsd.org/cgi/man.cgi?query=sysexits
+#library('exit_codes');
+
+/// The command was used incorrectly.
+final USAGE = 64;
+
+/// The input data was incorrect.
+final DATA = 65;
+
+/// An input file did not exist or was unreadable.
+final NO_INPUT = 66;
+
+/// The user specified did not exist.
+final NO_USER = 67;
+
+/// The host specified did not exist.
+final NO_HOST = 68;
+
+/// A service is unavailable.
+final UNAVAILABLE = 69;
+
+/// An internal software error has been detected.
+final SOFTWARE = 70;
+
+/// An operating system error has been detected.
+final OS = 71;
+
+/// Some system file did not exist or was unreadable.
+final OS_FILE = 72;
+
+/// A user-specified output file cannot be created.
+final CANT_CREATE = 73;
+
+/// An error occurred while doing I/O on some file.
+final IO = 74;
+
+/// Temporary failure, indicating something that is not really an error.
+final TEMP_FAIL = 75;
+
+/// The remote system returned something invalid during a protocol exchange.
+final PROTOCOL = 76;
+
+/// The user did not have sufficient permissions.
+final NO_PERM = 77;
+
+/// Something was unconfigured or mis-configured.
+final CONFIG = 78;