Add new exception for JSON inverter models

[klaasnicolaas]

Tests have shown that inverters sometimes give 0 values for both day and total production, which results in peaks in the Home Assistant Energy dashboard. This PR adds a new exception for those using the JSON source.

• Add exception for JSON source
• Add pytest for covering the new code

[codecov]

Codecov Report

Merging #31 (d98ec26) into master (cac98a5) will increase coverage by 0.45%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
+ Coverage   90.97%   91.42%   +0.45%     
==========================================
  Files           4        4              
  Lines         133      140       +7     
  Branches       19       21       +2     
==========================================
+ Hits          121      128       +7     
  Misses          9        9              
  Partials        3        3              

Impacted Files	Coverage Δ
omnikinverter/__init__.py	100.00% <100.00%> (ø)
omnikinverter/exceptions.py	100.00% <100.00%> (ø)
omnikinverter/models.py	100.00% <100.00%> (ø)

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