MLBEDSW-2567: CLI option to specify allocation alignment

Added the CLI option. Only applies to CPU tensors. Added an
AllocationError which is raised when Allocation fails.

Signed-off-by: Jacob Bohlin <jacob.bohlin@arm.com>
Change-Id: I89164dea3ac7b7add7bc40aec2ce8fe50600105d
diff --git a/ethosu/vela/errors.py b/ethosu/vela/errors.py
index 2c93fbc..1a30d54 100644
--- a/ethosu/vela/errors.py
+++ b/ethosu/vela/errors.py
@@ -52,6 +52,13 @@
         self.data = "Incorrect argument to CLI option: {} {}: {}".format(option, option_value, msg)
 
 
+class AllocationError(VelaError):
+    """Raised when allocation fails"""
+
+    def __init__(self, msg):
+        self.data = msg
+
+
 def OperatorError(op, msg):
     """Called when parsing an operator results in errors"""