[seabreeze] add comments about importing exceptions from backends

Add a comment to prevent direct import from backends.
It's not worth protecting against this by overcomplicating things.
Close #101.

Author: ap--

src/seabreeze/cseabreeze/c_seabreeze_wrapper.pyx

@@ -35,6 +35,8 @@ DEF _MAXBUFLEN = 32
 DEF _MAXDBUFLEN = 256
 
 
+# DO NOT DIRECTLY IMPORT EXCEPTIONS FROM HERE!
+# ALWAYS IMPORT FROM `seabreeze.spectrometers`
 class SeaBreezeError(Exception):
 
     _error_msgs = (

src/seabreeze/pyseabreeze/exceptions.py

@@ -1,11 +1,10 @@
-"""custom seabreeze exceptions
-
-"""
+"""custom seabreeze exceptions"""
+# DO NOT DIRECTLY IMPORT EXCEPTIONS FROM HERE!
+# ALWAYS IMPORT FROM `seabreeze.spectrometers`
 
 
 class SeaBreezeError(Exception):
     """SeaBreezeError base class"""
-
     pass