Add Useful Error Message for TVP Elem Types

[jbohanon]

Error message ErrorTypeSlice is misleading when returned as a result of the types of the elements of the slice not being structs. Added a more useful error message to elaborate on the particular error.

[codecov]

Codecov Report

Merging #719 (8cf1136) into master (e538731) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #719      +/-   ##
==========================================
- Coverage   71.29%   71.24%   -0.06%     
==========================================
  Files          24       24              
  Lines        5386     5386              
==========================================
- Hits         3840     3837       -3     
- Misses       1301     1303       +2     
- Partials      245      246       +1     

Impacted Files	Coverage Δ
tvp_go19.go	93.02% <100.00%> (ø)
tds.go	65.49% <0.00%> (-0.37%)	⬇️

---

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 e538731...8cf1136. Read the comment docs.