Log Rewards errors by default and add a feature flag for verbose logging (uplift to 1.25.x)

[brave-builds]

Uplift of #8598 and #8767
Resolves brave/brave-browser#15500 & brave/brave-browser#15661

Pre-approval checklist:

• You have tested your change on Nightly.
• This contains text which needs to be translated.
  • There are more than 7 days before the release.
  • I've notified folks in #l10n on Slack that translations are needed.
• The PR milestones match the branch they are landing to.

Pre-merge checklist:

• You have checked CI and the builds, lint, and tests all pass or are not related to your PR.

Post-merge checklist:

• The associated issue milestone is set to the smallest version that the changes is landed on.

[kjozwiak]

test-browser failed on Linux as per https://ci.brave.com/job/pr-brave-browser-pr8598_ksmith-rewards-log-errors_1.25.x-linux/1/testReport/. However, these are known intermittent issues/failures and unrelated to this specific PR.

linux / test-browser / BraveAdsBrowserTest/BraveAdsUpgradeBrowserTest.UpgradePath/PreferencesForVersion062WithRewardsEnabled_ForSupportedLocale_ForUnsupportedLocale_RewardsShouldBeEnabled_AdsShouldBeDisabled
linux / test-browser / RewardsPublisherBrowserTest.VisitRegisteredPublisher

Restarting iOS as dist failed as per https://ci.brave.com/job/pr-brave-browser-pr8598_ksmith-rewards-log-errors_1.25.x-ios/1/execution/node/187/log/. Hopefully it passes the second time around as we've been having issues with macOS & iOS.

18:59:41  error: the path does not point to a valid debug symbols file: /Users/jenkins/jenkins/workspace/pr-brave-browser-pr8598_ksmith-rewards-log-errors_1.25.x-ios/src/out/ios_Release_arm64/BraveRewards.dSYM

[kjozwiak]

We'll wait till brave/brave-browser#15661 gets addressed/fixed. Once the fix lands in Nightly, QA will run through it and get this uplifted into 1.25.x if everything looks good 👍

[kjozwiak]

post-init failed due to audit-deps warning(s) as per https://ci.brave.com/job/pr-brave-browser-pr8598_ksmith-rewards-log-errors_1.25.x-post-init/2/execution/node/144/log/. However, these are known intermittent issues and not related to this specific PR.

[kjozwiak]

test-browser failed on macOS as per https://ci.brave.com/job/pr-brave-browser-pr8598_ksmith-rewards-log-errors_1.25.x-macos/3/testReport/. However, this is a known intermittent failure and unrelated to this specific PR.

macos-x64 / test-browser / BraveAdsBrowserTest/BraveAdsUpgradeBrowserTest.UpgradePath/PreferencesForVersion063WithRewardsAndAdsEnabled_ForSupportedLocale_ForNewlySupportedLocale_RewardsShouldBeEnabled_AdsShouldBeDisabled

[kjozwiak]

Uplift into 1.25.x approved after deliberating with @brave/uplift-approvers. QA has verified the PR on Nightly as per #8598 (comment) & #8767 (comment).