[browser] Improve errors for failed asset download

[maraf]

Launching chrome ('/root/helix/work/correlation/chrome-linux/chrome') via playwright with args = --explicitly-allowed-ports=42135,--ignore-certificate-errors,--lang=en-US,--headless
[error] Failed to load resource: the server responded with a status of 404 (Not Found)
[error] Failed to load resource: the server responded with a status of 404 (Not Found)
[error] Failed to load resource: the server responded with a status of 404 (Not Found)
Waiting for page to load
[error] Error in mono_download_assets: [object Object]
[error] MONO_WASM: [object Object]
Error
    at Ke (http://127.0.0.1:42135/_framework/dotnet.0mv726uglb.js:4:23485)
    at http://127.0.0.1:42135/_framework/dotnet.0mv726uglb.js:4:8232
[ERROR] PageError: ErrnoError: ErrnoError
[ERROR] PageError: ErrnoError: ErrnoError
[ERROR] PageError: ErrnoError: ErrnoError
[ERROR] PageError: ErrnoError: ErrnoError
[ERROR] PageError: Error: Failed to start platform. Reason: [object Object]
    at qt (http://127.0.0.1:42135/_framework/blazor.webassembly.d97du072fb.js:1:55619)
Browser has been disconnected

https://helix.dot.net/api/2019-06-17/jobs/0b30fb76-baea-4452-8545-1f716e7c3ab8/workitems/NoWebcil-ST-Wasm.Build.Tests/console

[dotnet-policy-service]

Tagging subscribers to 'arch-wasm': @lewing, @pavelsavara
See info in area-owners.md if you want to be subscribed.

[pavelsavara]

Not too bad for CoreCLR

Uncaught Error: Failed to load resource 'System.Diagnostics.StackTrace.ycpk7he5s8.dll' 
from 'http://localhost:64186/_framework/System.Diagnostics.StackTrace.ycpk7he5s8.dll?v=42?v=42?v=42': Failed to load resource 'System.Diagnostics.StackTrace.ycpk7he5s8.dll' 
from 'http://localhost:64186/_framework/System.Diagnostics.StackTrace.ycpk7he5s8.dll?v=42?v=42?v=42' 
after multiple attempts. Last HTTP status: 500 Failed to fetch
    at Re (dotnet.js:5:15938)
    at async dotnet.js:5:14097
    at async be (dotnet.js:5:14017)
    at async Promise.all (:64186/index 2)
    at async Ve (dotnet.js:5:27218)
    at async main.js:77:5Caused by: Error: Failed to load resource 'System.Diagnostics.StackTrace.ycpk7he5s8.dll' from 'http://localhost:64186/_framework/System.Diagnostics.StackTrace.ycpk7he5s8.dll?v=42?v=42?v=42' after multiple attempts. Last HTTP status: 500 Failed to fetch
    at dotnet.js:5:16829
    at async Re (dotnet.js:5:15864)
    at async dotnet.js:5:14097
    at async be (dotnet.js:5:14017)
    at async Promise.all (:64186/index 2)
    at async Ve (dotnet.js:5:27218)
    at async main.js:77:5