scripts/stall-analyser: improve error messages on invalid input

and make sure to exit with failure status

Currently the program hits an internal error on
invalid input like: `Reactor stalled for 4 ms on shard 0.  Backtrace:`
that has no following backtrace.

Instead of emitting an obscure backtrace, like:
```
Traceback (most recent call last):
  File "/home/bhalevy/dev/seastar/./scripts/stall-analyser.py", line 461, in <module>
    main()
  File "/home/bhalevy/dev/seastar/./scripts/stall-analyser.py", line 443, in main
    render.process_trace(trace, t)
  File "/home/bhalevy/dev/seastar/./scripts/stall-analyser.py", line 156, in process_trace
    self.add_head(t, node)
  File "/home/bhalevy/dev/seastar/./scripts/stall-analyser.py", line 175, in add_head
    self.head.link_callee(t, n)
  File "/home/bhalevy/dev/seastar/./scripts/stall-analyser.py", line 104, in link_callee
    if n.addr in self.callees:
       ^^^^^^
AttributeError: 'NoneType' object has no attribute 'addr'
```

Print a formal error message that describes how valid
input should look like in 2 cases:
1. When no valid input is found
2. When a line with empty backtrace is found.

Signed-off-by: Benny Halevy <[email protected]>

Closes #2558

scripts/stall-analyser.py

@@ -394,6 +394,7 @@ def main():
     args = get_command_line_parser().parse_args()
     comment = re.compile(r'^\s*#')
     pattern = re.compile(r"Reactor stalled for (?P<stall>\d+) ms on shard (?P<shard>\d+).*Backtrace:")
+    expected_input_format = "Expected one or more lines ending with: 'Reactor stalled for <n> ms on shard <i>. Backtrace: <addr> [<addr> ...]'"
     address_threshold = int(args.address_threshold, 0)
     # map from stall time in ms to the count of the stall time
     tally = {}
@@ -434,12 +435,19 @@ def main():
         if address_threshold:
             trace = list(dropwhile(lambda addr: int(addr, 0) >= address_threshold, trace))
         if t >= args.tmin:
+            if not trace:
+                print(f"""Invalid input line: '{s.strip()}'
+{expected_input_format}
+Please run `stall-analyser.py --help` for usage instruction""", file=sys.stderr)
+                sys.exit(1)
             render.process_trace(trace, t)
 
     try:
         if not render:
-            print("No input data found. Please run `stall-analyser.py --help` for usage instruction")
-            sys.exit()
+            print(f"""No input data found.
+{expected_input_format}
+Please run `stall-analyser.py --help` for usage instruction""", file=sys.stderr)
+            sys.exit(1)
         if args.format == 'graph':
             print_command_line_options(args)
             print_stats(tally, args.tmin)