Stale/broken `from-browser` URLs triggers an error message that starts with "Error: Error: "

STR:
1. Visit https://profiler.firefox.com/from-browser/calltree/
(or, capture a profile, and don't upload it, and then quit and session-restore Firefox, including that tab. This results in a URL like that^ with a long suffix)
2. Look at the error message shown in the page and in the web console.

ACTUAL RESULTS:
Both the page and the Web Console show:
```
Error: Error: Could not find a profile for this tab.
```

EXPECTED RESULTS:
No double-"Error" prefix.

If it's helpful, here's full message from Web Console, including the error message and the backtrace. Note that the error is attributed to something in `web-channel.ts`, called into from `receive-profile.ts`.
```
Error: Error: Could not find a profile for this tab.
a web-channel.ts:248
AT receive-profile.ts:1061
Redux 2
MR receive-profile.ts:1398
Redux 2
_processInitialUrls UrlManager.tsx:123
componentDidMount UrlManager.tsx:208
React 44
Kk scheduler.production.js:151
(Async: EventHandlerNonNull)
lz scheduler.production.js:200
n chunk-FFESA2B5.js:1
cz index.js:4
n chunk-FFESA2B5.js:1
NU React
n chunk-FFESA2B5.js:1
RU React
n chunk-FFESA2B5.js:1
index.tsx:18
```

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-1335)

Reproduce the stale from-browser URL and inspect web-channel.ts around line 248 and receive-profile.ts around lines 1061 and 1398, following the stack trace from UrlManager.tsx. Confirm the error is formatted once in both the page and Web Console, without the duplicated "Error: " prefix.

Written by the indexing model from the issue text.