Adds error logic for one-time tip

[NejcZdovc]

Resolves brave/brave-browser#3052

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

• enable rewards
• claim grant
• go to verified site
• open panel
• select amount in donation banner
• (when you will click send tip you need to turn internet off immediately)
• click send tip
• disable internet
• make sure that you see error in the panel
  

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[davidtemkin]

@NejcZdovc please change text to "Unable to sent your tip. Please try again later."

[NejcZdovc]

@davidtemkin done

[ryanml]

++ thanks for change