[builder] migrate builder exceptions to separate module

The builder exceptions should be shared between different implementations of the
builder interface. This patch migrates them to a separate module
`dune.generator.exceptions`. For convenience they are also imported into
`dune.generator` itself.

CMakeLists.txt

@@ -2,5 +2,6 @@ add_python_targets(generator
   __init__
   algorithm
   builder
+  exceptions
   generator
   )

__init__.py

 import os
 
+from .exceptions import CompileError, ConfigurationError
 from .builder import Builder
 
 env_force = os.environ.get('DUNE_FORCE_BUILD', 'FALSE').upper()

builder.py

@@ -9,20 +9,12 @@ import sys
 
 from dune.common import comm
 from dune.common.compatibility import buffer_to_str, isString, reload_module
+from dune.generator.exceptions import CompileError, ConfigurationError
 import dune.common.module
 
 logger = logging.getLogger(__name__)
 
 class Builder:
-    class CompileError(Exception):
-        '''raise this when there's a problem compiling an extension module'''
-        def __init__(self, error):
-            Exception.__init__(self,error)
-    class ConfigurationError(Exception):
-        '''raise this when there's a problem with the configuration of dune-py'''
-        def __init__(self, error):
-            Exception.__init__(self,error)
-
     def __init__(self, force=False):
         self.force = force
 
@@ -52,7 +44,7 @@ class Builder:
         logger.debug(buffer_to_str(stdout))
         if cmake.returncode > 0:
             logger.error(buffer_to_str(stderr))
-            raise self.CompileError(buffer_to_str(stderr))
+            raise CompileError(buffer_to_str(stderr))
 
     def load(self, moduleName, source, pythonName):
         module = sys.modules.get("dune.generated." + moduleName)

exceptions.py

+from __future__ import absolute_import, division, print_function, unicode_literals
+
+class CompileError(Exception):
+    '''raise this when there's a problem compiling an extension module'''
+    def __init__(self, error):
+        Exception.__init__(self,error)
+class ConfigurationError(Exception):
+    '''raise this when there's a problem with the configuration of dune-py'''
+    def __init__(self, error):
+        Exception.__init__(self,error)