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/errors.py b/ethosu/vela/errors.py
new file mode 100644
index 0000000..efe64d5
--- /dev/null
+++ b/ethosu/vela/errors.py
@@ -0,0 +1,48 @@
+# Copyright (C) 2020 Arm Limited or its affiliates. All rights reserved.
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the License); you may
+# not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an AS IS BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# Description:
+# Defines custom exceptions.
+
+
+class VelaError(Exception):
+    """Base class for vela exceptions"""
+
+    def __init__(self, data):
+        self.data = data
+
+    def __str__(self):
+        return repr(self.data)
+
+
+class InputFileError(VelaError):
+    """Raised when reading the input file results in errors"""
+
+    def __init__(self, file_name, msg):
+        self.data = "Error reading {}: {}".format(file_name, msg)
+
+
+class UnsupportedFeatureError(VelaError):
+    """Raised when the input file uses non-supported features that cannot be handled"""
+
+    def __init__(self, data):
+        self.data = "The input file uses a feature that is currently not supported: {}".format(data)
+
+
+class OptionError(VelaError):
+    """Raised when an incorrect command line option is used"""
+
+    def __init__(self, option, option_value, msg):
+        self.data = "Incorrect argument: {} {}: {}".format(option, option_value, msg)