Add an error message when using Lark.save() when parser!='lalr'

[erezsh]

Addresses an oversight raised in issues #1323 and #788

[codecov-commenter]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.02% ⚠️

Comparison is base (656334c) 89.22% compared to head (ae6f43a) 89.21%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1328      +/-   ##
==========================================
- Coverage   89.22%   89.21%   -0.02%     
==========================================
  Files          51       51              
  Lines        7595     7597       +2     
==========================================
+ Hits         6777     6778       +1     
- Misses        818      819       +1     

Flag	Coverage Δ
unittests	89.21% <50.00%> (-0.02%)	⬇️

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Files Changed	Coverage Δ
lark/lark.py	85.62% <50.00%> (-0.23%)	⬇️

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