More detailed error output on corrupt UID mapping files

This function will need much more "robustifying", but the very least we
can do is to print the file name and line that are giving trouble.

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

offlineimap/folder/UIDMaps.py

@@ -51,7 +51,11 @@ class MappingFolderMixIn:
                 line = file.readline()
                 if not len(line):
                     break
-                line = line.strip()
+                try:
+                    line = line.strip()
+                except ValueError:
+                    raise Exception("Corrupt line '%s' in UID mapping file '%s'" \
+                                        %(line, mapfilename))
                 (str1, str2) = line.split(':')
                 loc = long(str1)
                 rem = long(str2)