MLBEDSW-2066 Improve Exception messages

Minor refactoring to use fstrings.
Improve Error classes to correctly inherit the base class.
Use existing exception classes instead of plain exceptions where it
makes sense.

Signed-off-by: Michael McGeagh <michael.mcgeagh@arm.com>
Change-Id: I0941c04e91010da1db77299517a8e2d896371e77
diff --git a/ethosu/vela/vela.py b/ethosu/vela/vela.py
index d27eef0..bfc76ec 100644
--- a/ethosu/vela/vela.py
+++ b/ethosu/vela/vela.py
@@ -55,7 +55,7 @@
     nng = model_reader.read_model(input_name, model_reader_options)
 
     if not nng:
-        raise InputFileError(input_name, "input file could not be read")
+        raise InputFileError(input_name, "Input file could not be read")
 
     if compiler_options.verbose_operators:
         nng.print_operators()
@@ -364,7 +364,7 @@
     if args.config is not None:
         for filename in args.config:
             if not os.access(filename, os.R_OK):
-                raise InputFileError(filename, "File not found or is not readable.")
+                raise InputFileError(filename, "File not found or is not readable")
 
     sys.setrecursionlimit(args.recursion_limit)