Improve setallowance UX

MERGE REQUEST

Overview

This is a pretty basic set of changes and low priority overall. I undertook this because I was a little sick and this was easy work.

The siac renter setallowance user experience has been vastly improved.

Changes made:

1. Some inputs did not accept a space between the value and the unit. Now all inputs accept spaces.
2. Accepted units are displayed when the unit is a filesize (like we were already doing for other units)
3. The program will no longer terminate on an input error, giving the user a chance to correct his input
4. A newline is printed between each step making them easier to distinguish visually

Example for Visual changes (ie Screenshot)

https://asciinema.org/a/aZ4xtqs5TVvZAbqkZaHuhy4Rk

Issues Closed

#3832 (closed)

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods, or updating methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to work flow
• Issue added to Sia-UI repo for new supporting features
• Changelog updated