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/compiler_driver.py b/ethosu/vela/compiler_driver.py
index 6c7fdc1..78d7f12 100644
--- a/ethosu/vela/compiler_driver.py
+++ b/ethosu/vela/compiler_driver.py
@@ -267,9 +267,9 @@
                 alloc_results.append(alloc_success)
             if not alloc_results[-1]:
                 raise VelaError(
-                    "Sram limit {} bytes, has been exceeded by the scratch fast tensor. "
+                    f"Sram limit {arch.sram_size} bytes, has been exceeded by the scratch fast tensor. "
                     "Increasing the value of --weight-estimation-scaling may help to resolve the issue. "
-                    "See OPTIONS.md for more information.".format(arch.sram_size)
+                    "See OPTIONS.md for more information"
                 )
         else:
             tensor_allocation.allocate_tensors(