fix[ux]: improve error message for bad hex literals

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

improve the error message when a hex literal starts with `0X`. it is
accepted by the parser, but prior to this commit, would result in a
compiler panic. this commit validates the literal and provides a proper
error message.

Description for the changelog

Cute Animal Picture

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 46.17%. Comparing base (f7b8e93) to head (6fd62d9).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
vyper/ast/nodes.py	0.00%	3 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (f7b8e93) and HEAD (6fd62d9). Click for more details.

HEAD has 139 uploads less than BASE

Flag	BASE (f7b8e93)	HEAD (6fd62d9)
	140	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4244       +/-   ##
===========================================
- Coverage   91.39%   46.17%   -45.22%     
===========================================
  Files         109      109               
  Lines       15625    15632        +7     
  Branches     3435     3436        +1     
===========================================
- Hits        14280     7218     -7062     
- Misses        916     7859     +6943     
- Partials      429      555      +126     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[cyberthirst]

keep up the good work!