Auto merge of #5678 - mdamien:patch-1, r=jdm

The urllib version used a `FancyURLOpener` which use urllib. But urllib does not not handle proxies with SSL well.

For example, when adding the proxies to urllib.FancyURLOpener, I got this SSL error:

    IOError: [Errno socket error] [Errno 1] _ssl.c:510: error:140770FC:SSL routines:SSL23_GET_SERVER_HELLO:unknown protocol

So I switched the function to urllib2.

A better solution would be to use `requests` but I prefer to stay with the "no-dependency" approach.

For example, this was my first solution with `requests`:

```python
def download(desc, src, dst):
    print("Downloading(R) %s..." % desc)
    with open(dst, 'wb') as handle:
        response = requests.get(src, stream=True)

        if not response.ok:
            print("something went wrong downloading %s, :(" % desc)
            sys.exit(1)

        size = int(response.headers['content-length'])
        block_size = 1024
        recved = 0
        for block in response.iter_content(1024):
            recved += len(block)
            pct = recved * 100.0 / size
            print("\rDownloading %s: %5.1f%%" % (desc, pct), end="")
            if not block:
                break
            handle.write(block)
    print()
```

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python/servo/bootstrap_commands.py

@@ -6,7 +6,7 @@ import shutil
 import subprocess
 import sys
 import tarfile
-import urllib
+import urllib2
 
 from mach.decorators import (
     CommandArgument,
@@ -16,33 +16,40 @@ 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]
-
-    def report(count, bsize, fsize):
-        recved[0] += bsize
-        pct = recved[0] * 100.0 / fsize
-        print("\rDownloading %s: %5.1f%%" % (desc, pct), end="")
-        sys.stdout.flush()
-
     print("Downloading %s..." % desc)
     dumb = (os.environ.get("TERM") == "dumb") or (not sys.stdout.isatty())
-    PanickyUrlOpener().retrieve(src, dst, None if dumb else report)
-    if not dumb:
-        print()
 
+    try: 
+        resp = urllib2.urlopen(src)
+        fsize = int(resp.info().getheader('Content-Length').strip())
+        recved = 0
+        chunk_size = 8192
+
+        with open(dst, 'wb') as fd:
+            while True:
+                chunk = resp.read(chunk_size)
+                if not chunk: break
+                recved += len(chunk)
+                if not dumb:
+                    pct = recved * 100.0 / fsize
+                    print("\rDownloading %s: %5.1f%%" % (desc, pct), end="")
+                    sys.stdout.flush()
+                fd.write(chunk)
+
+        if not dumb:
+            print()
+    except urllib2.HTTPError, e:
+        print("Download failed (%d): %s - %s" % (e.code, e.reason, src))
+
+        cpu_type = subprocess.check_output(["uname", "-m"]).strip().lower()
+        if e.code == 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 extract(src, dst, movedir=None):
     tarfile.open(src).extractall(dst)