vela: Fix block config override issue.

 - A blanket exception was preventing block config overrides from being used,
   from either code or command line.

Change-Id: I1a7aa7771e077bcdb66886a6b637d099ae43d732
Signed-off-by: Tim Hall <tim.hall@arm.com>
diff --git a/ethosu/vela/shared_buffer_allocation.py b/ethosu/vela/shared_buffer_allocation.py
index 040f2a3..07637f3 100644
--- a/ethosu/vela/shared_buffer_allocation.py
+++ b/ethosu/vela/shared_buffer_allocation.py
@@ -22,7 +22,7 @@
 from .architecture_features import Kernel
 from .architecture_features import SharedBufferArea
 from .architecture_features import SHRAMElements
-from .errors import OptionError
+from .errors import VelaError
 from .operation import NpuBlockType
 
 
@@ -167,11 +167,9 @@
 
     if arch.override_block_config:
         config = alloc.try_block(arch.override_block_config)
-        raise OptionError(
-            "--force-block-config",
-            str(arch.override_block_config),
-            "This forced block config value cannot be used; it is not compatible",
-        )
+        if config is None:
+            raise VelaError("Block config override '{0}' cannot be allocated".format(arch.override_block_config) )
+        return [config]
 
     # Constrain the search space if the OFM is smaller than the max block size
     # - Add other block search constraints here if required