Properly handle querying the bits when the bits have not been set

[peter-meemo]

Apple returns a bare string in the response with a 200 http code in this case. Since the master code only checks for error if the code is not 200, it will try to parse the bare string as json and die. The README states

// Note that SDK returns ErrBitStateNotFound error if no bits found

This PR creates and returns that error.

[codecov]

Codecov Report

Merging #12 (09986da) into master (ba52372) will decrease coverage by 4.06%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
- Coverage   92.14%   88.07%   -4.07%     
==========================================
  Files           9        9              
  Lines         140      151      +11     
==========================================
+ Hits          129      133       +4     
- Misses          7       12       +5     
- Partials        4        6       +2     

Impacted Files	Coverage Δ
error.go	86.36% <40.00%> (-13.64%)	⬇️
query.go	75.86% <44.44%> (-11.10%)	⬇️

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[rinchsan]

@peter-meemo
Thank you for your work!
I will tidy some codes, add some tests in a new branch, and release a new minor version with this bug fixes as soon as possible.