Warn in CLI when --end precedes --start

An inverted window is a legal but empty range, so the request still runs;
at the CLI it is almost always a typo, and the silent empty output gives
no hint why.

Signed-off-by: cwasicki <126617870+cwasicki@users.noreply.github.com>

Author: cwasicki

RELEASE_NOTES.md

@@ -10,7 +10,7 @@
 
 ## New Features
 
-<!-- Here goes the main new features and examples or instructions on how to use them -->
+- The CLI now warns when `--end` is before `--start`. The request still runs, but the inverted range returns no data, so the warning flags what is almost always a typo.
 
 ## Bug Fixes
 

src/frequenz/client/reporting/cli/__main__.py

@@ -107,6 +107,13 @@ def main() -> None:
         default=None,
     )
     args = parser.parse_args()
+    # An inverted window is a legal but empty range; warn since at the CLI it is
+    # almost always a typo.
+    if args.start and args.end and args.end < args.start:
+        print(
+            "warning: --end is before --start; no data will be returned",
+            file=sys.stderr,
+        )
     asyncio.run(
         run(
             microgrid_id=args.mid,