lfs: narrow the exceptions that trigger a transfer retry

The retries were added to workaround TCP RESETs in fb-experimental fc8c131314a9.
I have no idea if that's been debugged yet, but this wide net caught local I/O
errors, bad hostnames and other things that shouldn't be retried.  The next
patch will validate objects as they are uploaded, and there's no need to retry
those errors.

The spec[1] does mention that certain http errors can be retried, including 500.
But let's work through the corruption detection issues first.

[1] https://github.com/git-lfs/git-lfs/blob/master/docs/api/batch.md

hgext/lfs/blobstore.py

@@ -10,6 +10,7 b' from __future__ import absolute_import'
 import json
 import os
 import re
+import socket
 
 from mercurial.i18n import _
 
@@ -286,7 +287,7 b' class _gitlfsremote(object):'
                         self._basictransfer(obj, action, localstore)
                         yield 1, obj.get('oid')
                         break
-                    except Exception as ex:
+                    except socket.error as ex:
                         if retry > 0:
                             if self.ui.verbose:
                                 self.ui.write(

tests/test-lfs-test-server.t

@@ -146,12 +146,7 b' Test a corrupted file upload'
   pushing to ../repo1
   searching for changes
   lfs: uploading e659058e26b07b39d2a9c7145b3f99b41f797b6621c8076600e9cb7ee88291f0 (17 bytes)
-  lfs: failed: LfsRemoteError('HTTP error: HTTP Error 500: Internal Server Error (oid=e659058e26b07b39d2a9c7145b3f99b41f797b6621c8076600e9cb7ee88291f0, action=upload)',) (remaining retry 5)
-  lfs: failed: LfsRemoteError('HTTP error: HTTP Error 404: Not Found (oid=e659058e26b07b39d2a9c7145b3f99b41f797b6621c8076600e9cb7ee88291f0, action=upload)',) (remaining retry 4)
-  lfs: failed: LfsRemoteError('HTTP error: HTTP Error 404: Not Found (oid=e659058e26b07b39d2a9c7145b3f99b41f797b6621c8076600e9cb7ee88291f0, action=upload)',) (remaining retry 3)
-  lfs: failed: LfsRemoteError('HTTP error: HTTP Error 404: Not Found (oid=e659058e26b07b39d2a9c7145b3f99b41f797b6621c8076600e9cb7ee88291f0, action=upload)',) (remaining retry 2)
-  lfs: failed: LfsRemoteError('HTTP error: HTTP Error 404: Not Found (oid=e659058e26b07b39d2a9c7145b3f99b41f797b6621c8076600e9cb7ee88291f0, action=upload)',) (remaining retry 1)
-  abort: HTTP error: HTTP Error 404: Not Found (oid=e659058e26b07b39d2a9c7145b3f99b41f797b6621c8076600e9cb7ee88291f0, action=upload)!
+  abort: HTTP error: HTTP Error 500: Internal Server Error (oid=e659058e26b07b39d2a9c7145b3f99b41f797b6621c8076600e9cb7ee88291f0, action=upload)!
   [255]
 
 Check error message when the remote missed a blob: