improvement: assert descriptions in Crowdfund finalize() and participate()

[nfwsncked]

What I did

Changed "deadline has passed" to "deadline is already overdue" in participate assert description
Changed "deadline not met (yet)" to "deadline is already overdue" in finalize assert description
Changed "the goal has not been reached" to "goal has not been reached" in finalize assert description

How I did it

It was fast and easy, just some typos or not specific descriptions

How to verify it

https://raw.githubusercontent.com/vyperlang/vyper/master/examples/crowdfund.vy
https://raw.githubusercontent.com/nfwsncked/vyper/master/examples/crowdfund.vy

Commit message

improvement: assert descriptions in Crowdfund.finalize() and Crowdfund.p…

Description for the changelog

Better assertion descriptions in Crowdfund example contract

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[codecov-commenter]

Codecov Report

Merging #3064 (5b4ae55) into master (5d10ea0) will increase coverage by 0.03%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master    #3064      +/-   ##
==========================================
+ Coverage   89.50%   89.54%   +0.03%     
==========================================
  Files          80       80              
  Lines       11342    11342              
  Branches     2553     2553              
==========================================
+ Hits        10152    10156       +4     
+ Misses        797      793       -4     
  Partials      393      393              

see 1 file with indirect coverage changes

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