Prevent pants.bootstrap failure when crawl a page and get BadStatusLine

Fixes GH-210
Use HTTPException instead of BadStatusLine
Have http raise a FetchError that crawler can catch

instead of having http leak out exceptions from urllib and httplib
Document definition and use of http.FetchError
test_connect_timeout_using_open: Check for FetchError

instead of urllib_error.URLError
Expose FetchError in twitter/common/python/http/__init__.py
Merge pull request #211 from msabramo/prevent_pants_bootstrap_failure_on_BadStatusLine

Prevent pants.bootstrap failure when crawl a page and get BadStatusLine