Improve BCP395 error message

[shenglol]

Closes #13672.

Microsoft Reviewers: Open in CodeFlow

[github-actions]

Test this change out locally with the following install scripts (Action run 9070755190)

VSCode

• Mac/Linux

  bash <(curl -Ls https://aka.ms/bicep/nightly-vsix.sh) --run-id 9070755190

• Windows

  iex "& { $(irm https://aka.ms/bicep/nightly-vsix.ps1) } -RunId 9070755190"

Azure CLI

• Mac/Linux

  bash <(curl -Ls https://aka.ms/bicep/nightly-cli.sh) --run-id 9070755190

• Windows

  iex "& { $(irm https://aka.ms/bicep/nightly-cli.ps1) } -RunId 9070755190"

[github-actions]

Dotnet Test Results

    66 files   -     33      66 suites   - 33   22m 13s ⏱️ - 10m 19s
10 890 tests  -     20  10 889 ✅  -     20  1 💤 ±0  0 ❌ ±0 
25 676 runs   - 12 834  25 674 ✅  - 12 833  2 💤  - 1  0 ❌ ±0 

Results for commit 9bb639f. ± Comparison against base commit cd8ad8a.