auto merge of #3894 : awestroke/servo/feauture-warn-32bit-bootstrap, r=jdm

Also hints that 32bit snapshots are unavailable on failure

Resolves #3739 

![screenshot from 2014-11-05 13 42 39](https://cloud.githubusercontent.com/assets/1098408/4917781/6edc0dd2-64e9-11e4-98bd-dbd1829ee913.png)

python/servo/bootstrap_commands.py

@@ -16,6 +16,17 @@ from mach.decorators import (
 
 from servo.command_base import CommandBase, cd, host_triple
 
+class PanickyUrlOpener(urllib.FancyURLopener):
+  def http_error_default(self, url, fp, errcode, errmsg, headers):
+    print("Download failed (%d): %s - %s" % (errcode, errmsg, url))
+
+    cpu_type = subprocess.check_output(["uname", "-m"]).strip().lower()
+    if errcode == 404 and cpu_type in ["i386", "i486", "i686", "i768", "x86"]:
+        # i686
+        print("Note: Servo does not currently bootstrap 32bit snapshots of Rust")
+        print("See https://github.com/servo/servo/issues/3899")
+
+    sys.exit(1)
 
 def download(desc, src, dst):
     recved = [0]
@@ -28,7 +39,7 @@ def download(desc, src, dst):
 
     print("Downloading %s..." % desc)
     dumb = os.environ.get("TERM") == "dumb"
-    urllib.urlretrieve(src, dst, None if dumb else report)
+    PanickyUrlOpener().retrieve(src, dst, None if dumb else report)
     if not dumb:
         print()