fix: KeyError when handling some RPC exceptions

[iamdefinitelyahuman]

What I did

Raise a ValueError when the output used to create a VirtualMachineError cannot be properly parsed. This should fix issues with KeyError when Geth complains about e.g. a hardware wallet not being unlocked, or a lack of peers.

[codecov-commenter]

Codecov Report

Merging #550 into master will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #550      +/-   ##
==========================================
- Coverage   88.19%   88.17%   -0.03%     
==========================================
  Files          56       56              
  Lines        6304     6306       +2     
  Branches     1392     1393       +1     
==========================================
  Hits         5560     5560              
- Misses        514      515       +1     
- Partials      230      231       +1     

Impacted Files	Coverage Δ
brownie/exceptions.py	84.15% <0.00%> (-1.71%)	⬇️

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