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/weight_compressor.py b/ethosu/vela/weight_compressor.py
index fce17d1..188b16a 100644
--- a/ethosu/vela/weight_compressor.py
+++ b/ethosu/vela/weight_compressor.py
@@ -452,18 +452,14 @@
                 for weight_scale in weight_scales
             ]
         else:
-            raise UnsupportedFeatureError(
-                "Compression of {} is not implemented; tensor: {}".format(ifm_dtype, tens.name)
-            )
+            raise UnsupportedFeatureError(f"Compression of {ifm_dtype} is not implemented; Tensor: '{tens.name}'")
     else:
         if ifm_dtype == DataType.uint8:
             scales = [np.double(ifm_scale * weight_scale * 0x3000) for weight_scale in weight_scales]
         elif ifm_dtype == DataType.int8 or ifm_dtype == DataType.int16:
             scales = [(np.double(ifm_scale * 0x3000) * np.double(weight_scale)) for weight_scale in weight_scales]
         else:
-            raise UnsupportedFeatureError(
-                "Compression of {} is not implemented; tensor: {}".format(ifm_dtype, tens.name)
-            )
+            raise UnsupportedFeatureError(f"Compression of {ifm_dtype} is not implemented; Tensor: '{tens.name}'")
 
     # quantise all of the weight scales into (scale_factor, shift)
     if ifm_dtype == DataType.int16: