Add Error methods to get SQLite error values

[nvzqz]

Enables writing shorter code such as:

match error.sqlite_error_code() {
    Some(rusqlite::ffi::ErrorCode::ReadOnly) => // ...
    // ...
}

[codecov]

Codecov Report

Merging #1158 (b0993a1) into master (33e5f12) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1158      +/-   ##
==========================================
- Coverage   78.89%   78.82%   -0.08%     
==========================================
  Files          48       48              
  Lines        6028     6034       +6     
==========================================
  Hits         4756     4756              
- Misses       1272     1278       +6     

Impacted Files	Coverage Δ
src/error.rs	14.28% <0.00%> (-0.87%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 33e5f12...b0993a1. Read the comment docs.