scripts: west: improve error message on missing cache

In the west flash/debug commands, if the user gives an invalid build
directory, they'll get a stack trace instead of a helpful error
message when the cache can't be built.

Fix that.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>

scripts/west_commands/run_common.py

@@ -171,7 +171,10 @@ def do_run_common(command, args, runner_args, cached_runner_var):
     # make sure it supports the command.
 
     cache_file = path.join(build_dir, args.cmake_cache or cmake.DEFAULT_CACHE)
-    cache = cmake.CMakeCache(cache_file)
+    try:
+        cache = cmake.CMakeCache(cache_file)
+    except FileNotFoundError:
+        log.die('no CMake cache found (expected one at {})'.format(cache_file))
     board = cache['CACHED_BOARD']
     available = cache.get_list('ZEPHYR_RUNNERS')
     if not available: