Fallback for BTC transaction fee estimation

[Imamah-Zafar]

Component

Send BTC

Severity

Choose one:

• Major (Unable to proceed on a journey)

Summary

If Xverse API goes down, app is unable to estimate BTC transaction fees properly. It should fail gracefully and allow users to customize the fee.

[edit]: Will also include #383 (comment)
#383 (comment)

Reproduce steps

1. Change Xverse API URL to an unreachable URL
2. Try to send BTC

Expected result

BTC fee estimation should fail, but fallback to a default amount and allow users to set it.

Actual result

Cannot send BTC

Remark

(Optional)

[DuskaT021]

This pr should also include fixes for this issues: #383 (comment) and #383 (comment)