Explicit error messages instead of `nil` values everywhere

[gudzpoz]

Fixes #18.

1. No arbitrary nil values any more. Yes, any exception will interrupt the whole program. Use pcall for simplistic try-&-catch.
2. For JFunction, push an error message and return -1 to indicate an error. See the Javadoc for more info.

[codecov-commenter]

Codecov Report

Merging #20 (fe2cca7) into main (2c70a1c) will increase coverage by 0.35%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main      #20      +/-   ##
============================================
+ Coverage     91.63%   91.99%   +0.35%     
- Complexity      416      423       +7     
============================================
  Files            24       24              
  Lines           909      949      +40     
  Branches        116      122       +6     
============================================
+ Hits            833      873      +40     
  Misses           56       56              
  Partials         20       20              

Impacted Files	Coverage Δ
...uajava/src/main/java/party/iroiro/luajava/Lua.java	100.00% <ø> (ø)
...ava/src/main/java/party/iroiro/luajava/JuaAPI.java	100.00% <100.00%> (ø)

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