Provide more meaningful error messages

[glifchits]

Trying to integrate this module into an app I get:

  Error: Unsupported geometry type
      at features.forEach (/app/node_modules/mvt-encoder/tileEncoder.js:139:32)
      at Array.forEach (<anonymous>)
      at encodeFeatures (/app/node_modules/mvt-encoder/tileEncoder.js:138:12)

(provided EWKT from PostGIS, not a WKT)

or

  TypeError: Cannot read property 'count' of undefined
      at encodeGeometry (/app/node_modules/mvt-encoder/tileEncoder.js:74:45)
      at features.forEach (/app/node_modules/mvt-encoder/tileEncoder.js:144:17)
      at Array.forEach (<anonymous>)

(probably because I provided a MULTIPOLYGON)

Test cases to check:

• Unsupported input geometry (require WKT)
• Unsupported geometry during encoding (only support POLYGON for now)
• Coordinates not scaled to tile extent (forgot to translate them)