MLBEDSW-2067: added custom exceptions

Added custom exceptions to handle different types of input errors.

Also performed minor formatting changes using flake8/black.

Change-Id: Ie5b05361507d5e569aff045757aec0a4a755ae98
Signed-off-by: Louis Verhaard <louis.verhaard@arm.com>
diff --git a/ethosu/vela/model_reader.py b/ethosu/vela/model_reader.py
index d1cdc9b..6deb253 100644
--- a/ethosu/vela/model_reader.py
+++ b/ethosu/vela/model_reader.py
@@ -15,6 +15,9 @@
 # limitations under the License.
 # Description:
 # Dispatcher for reading a neural network model.
+from . import tflite_reader
+from .errors import InputFileError
+from .errors import VelaError
 
 
 class ModelReaderOptions:
@@ -29,15 +32,17 @@
 
 def read_model(fname, options, feed_dict={}, output_node_names=[], initialisation_nodes=[]):
     if fname.endswith(".tflite"):
-        from . import tflite_reader
-
-        nng = tflite_reader.read_tflite(
-            fname,
-            options.batch_size,
-            feed_dict=feed_dict,
-            output_node_names=output_node_names,
-            initialisation_nodes=initialisation_nodes,
-        )
+        try:
+            return tflite_reader.read_tflite(
+                fname,
+                options.batch_size,
+                feed_dict=feed_dict,
+                output_node_names=output_node_names,
+                initialisation_nodes=initialisation_nodes,
+            )
+        except VelaError as e:
+            raise e
+        except Exception as e:
+            raise InputFileError(fname, str(e))
     else:
-        assert 0, "Unknown model format"
-    return nng
+        raise InputFileError(fname, "Unknown input file format. Only .tflite files are supported")