Warn about insufficient funds at disposal to pay the transaction fee when configuring earning

[Radiokot]

Purpose

Fix not warning about insufficient funds at disposal to pay the transaction fee when configuring earning.

Changes

• Use correct at disposal value when checking the amount
• Add specific error message for the case

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.