Stop passing raw Exceptions as the reasons for ironic Image exceptions

When an exception occured in the ironic image service code, the raw
exception was passed into the IronicException as the reason. The
ironic exception logic then attempted to converted it to json which
fails and then falls back to converting it to string. This patch
preemptively converts the Exception to string to avoid that first
failure.

Change-Id: Id3a166a74505c142d7217217882246a64f625eb8

ironic/common/image_service.py

@@ -118,7 +118,7 @@ class HttpImageService(BaseImageService):
                              "HEAD request.") % response.status_code)
         except requests.RequestException as e:
             raise exception.ImageRefValidationFailed(image_href=output_url,
-                                                     reason=e)
+                                                     reason=six.text_type(e))
         return response
 
     def download(self, image_href, image_file):
@@ -143,7 +143,7 @@ class HttpImageService(BaseImageService):
                 shutil.copyfileobj(input_img, image_file, IMAGE_CHUNK_SIZE)
         except (requests.RequestException, IOError) as e:
             raise exception.ImageDownloadFailed(image_href=image_href,
-                                                reason=e)
+                                                reason=six.text_type(e))
 
     def show(self, image_href):
         """Get dictionary of image properties.
@@ -235,7 +235,7 @@ class FileImageService(BaseImageService):
                                       0, filesize)
         except Exception as e:
             raise exception.ImageDownloadFailed(image_href=image_href,
-                                                reason=e)
+                                                reason=six.text_type(e))
 
     def show(self, image_href):
         """Get dictionary of image properties.