improvements

[rubenguc]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/kumawallet/extension/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

• Error showed on send tx:
  

What is the new behavior?

• after:
  

• Added one step about signing messages in contributing file

Does this PR introduce a breaking change?

• Yes
• No

Other information