chore: improve exceptions for implements syntax

[tserg]

What I did

Fix #3297, fix #3298.

How I did it

For 3297, change the exception from UnexpectedNodeType to StructureException.
For 3298, as suggested in issue.

How to verify it

See tests

Commit message

chore: improve exceptions for `implements` syntax

Description for the changelog

Improve exceptions for implements syntax

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[codecov-commenter]

Codecov Report

Merging #3304 (b7d7d88) into master (dd4d0cc) will increase coverage by 0.01%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #3304      +/-   ##
==========================================
+ Coverage   88.87%   88.89%   +0.01%     
==========================================
  Files          84       84              
  Lines       10594    10595       +1     
  Branches     2211     2212       +1     
==========================================
+ Hits         9415     9418       +3     
+ Misses        770      769       -1     
+ Partials      409      408       -1     

Impacted Files	Coverage Δ
vyper/ast/nodes.py	94.10% <100.00%> (+0.31%)	⬆️
vyper/semantics/analysis/module.py	86.29% <100.00%> (+0.06%)	⬆️

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