Improve error message for IPFS gateway without Origin isolation

[lidel]

UX problem

A check was introduced in #16999 that blocks use of gateways without Origin isolation:

The error message "Invalid gateway address" is bit confusing, because ipfs.io is a valid gateway, it is just insecure when used for loading websites that expect Origin isolation.

Proposed fix

I suggest renaming the error message to something long in the lines of "Only a valid IPFS gateway with Origin isolation enabled can be used in Brave"

cc @spylogsster

[stephendonner]

Verified PASSED using

Brave	1.32.7 Chromium: 94.0.4606.50 (Official Build) nightly (x86_64)
Revision	0c1ac2c4842a4746c27c937c1a0453f98da1a972-refs/branch-heads/4606@{#1049}
OS	macOS Version 11.6 (Build 20G165)

Steps:

1. new profile
2. launch Brave
3. open brave://settings/ipfs
4. click on the Change button next to IPFS public gateway address
5. enter https://ipfs.io
6. click Submit

Confirmed we now see Only a valid IPFS gateway with Origin isolation enabled can be used in Brave as the more-appropriate error message.