Properly re-raise exception to save original tracebacks

We usually mutate some exceptions to OfflineImapError() and it is
a whole lot better if such exception will show up with the original
traceback, so all valid occurrences of such mutations were transformed
to the 3-tuple form of "raise".  Had also added coding guidelines
document where this re-raise strategy is documented.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/repository/IMAP.py

@@ -103,7 +103,8 @@ class IMAPRepository(BaseRepository):
             except Exception as e:
                 raise OfflineImapError("remotehosteval option for repository "\
                                        "'%s' failed:\n%s" % (self, e),
-                                       OfflineImapError.ERROR.REPO)
+                                       OfflineImapError.ERROR.REPO), \
+                      None, exc_info()[2]
             if host:
                 self._host = host
                 return self._host

offlineimap/repository/__init__.py

@@ -15,6 +15,8 @@
 #    along with this program; if not, write to the Free Software
 #    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 
+from sys import exc_info
+
 try:
     from configparser import NoSectionError
 except ImportError: #python2
@@ -66,14 +68,16 @@ class Repository(object):
         except NoSectionError as e:
             errstr = ("Could not find section '%s' in configuration. Required "
                       "for account '%s'." % ('Repository %s' % name, account))
-            raise OfflineImapError(errstr, OfflineImapError.ERROR.REPO)
+            raise OfflineImapError(errstr, OfflineImapError.ERROR.REPO), \
+                  None, exc_info()[2]
 
         try:
             repo = typemap[repostype]
         except KeyError:
             errstr = "'%s' repository not supported for '%s' repositories." \
                      % (repostype, reqtype)
-            raise OfflineImapError(errstr, OfflineImapError.ERROR.REPO)
+            raise OfflineImapError(errstr, OfflineImapError.ERROR.REPO), \
+                  None, exc_info()[2]
 
         return repo(name, account)