Add more descriptive error message when rule file not found.

[YamatoSecurity]

Right now when running html-server we get a generic Cannot open: error message when the rule file is not found, but it would be better to have more explanation: Could not open the sigma rule because the -r option was not used to specify the directory of sigma rules.

Ex:

./takajo html-server  -t ../hayabusa/timeline.jsonl
┏━━━━┳━━━┳┓┏━┳━━━┓ ┏┳━━━┓
┃┏┓┏┓┃┏━┓┃┃┃┏┫┏━┓┃ ┃┃┏━┓┃
┗┛┃┃┗┫┃ ┃┃┗┛┛┃┃ ┃┃ ┃┃┃ ┃┃
  ┃┃ ┃┗━┛┃┏┓┓┃┗━┛┣┓┃┃┃ ┃┃
  ┃┃ ┃┏━┓┃┃┃┗┫┏━┓┃┗┛┃┗━┛┃
  ┗┛ ┗┛ ┗┻┛┗━┻┛ ┗┻━━┻━━━┛
  by Yamato Security

html-report.sqlite already exists. It looks like you have already processed the JSONL file. Do you want to use this file (Y/n):
n
Started the html-report command

This command will create HTML summary reports for rules and computers with detections

Counting total lines. Please wait.

File: ../hayabusa/challenge1-verbose.jsonl (423.68 MB)
Total lines: 322,973

Scanning the Hayabusa timeline. Please wait.

100%|█████████████████████████| 322973/322973 [17.2s< 0.0s,  34.82k/sec]

Database file created.
You can access the HTML summary reports at http://localhost:8823
cannot open: