scripts: More explicit error in west_commands for NRF flashing

Instead of suggesting a python dep is missing, tell the user intelhex is
missing.

Signed-off-by: Félix Piédallu <felix@piedallu.me>
diff --git a/scripts/west_commands/runners/nrf_common.py b/scripts/west_commands/runners/nrf_common.py
index a183237..5578972 100644
--- a/scripts/west_commands/runners/nrf_common.py
+++ b/scripts/west_commands/runners/nrf_common.py
@@ -443,7 +443,7 @@
 
         self.ensure_output('hex')
         if IntelHex is None:
-            raise RuntimeError('one or more Python dependencies were missing; '
+            raise RuntimeError('Python dependency intelhex was missing; '
                                'see the getting started guide for details on '
                                'how to fix')
         self.hex_contents = IntelHex()