Add OfflineImapError class

This Exception can be thrown whenever a sync error occurs. It is used
for outputting sensible error messages to the user and it denotes a
"severity", it can tell offlineimap if we need to abort a message, a
folder, a repo sync, or whether we should indeed abort immediately.

The exception is not yet used in the code.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/error.py

@@ -0,0 +1,33 @@
+class OfflineImapError(Exception):
+    """An Error during offlineimap synchronization"""
+
+    class ERROR:
+        """Severity levels"""
+        MESSAGE  = 0
+        FOLDER   = 10
+        REPO     = 20
+        CRITICAL = 30
+
+    def __init__(self, reason, severity, errcode=None):
+        """
+        :param reason: Human readable string suitable for logging
+
+        :param severity: denoting which operations should be
+               aborted. E.g. a ERROR.MESSAGE can occur on a faulty
+               message, but a ERROR.REPO occurs when the server is
+               offline.
+
+         :param errcode: optional number denoting a predefined error
+               situation (which let's us exit with a predefined exit
+               value). So far, no errcodes have been defined yet.
+
+        :type severity: OfflineImapError.ERROR value"""
+        self.errcode  = errcode
+        self.severity = severity
+
+        # 'reason' is stored in the Exception().args tuple.
+        super(OfflineImapError, self).__init__(reason)
+
+    @property
+    def reason(self):
+        return self.args[0]