Corrected a typo in Tessellator#addQuadraticVertex()

Fixes #2648 - corrected a typo in a5cbb0e2
Fixed error reporting for bez/quad/curve vertices

When user calls bezierVertex()/quadraticVertex()/curveVertex() and does
not call beginShape()/beginShape(POLYGON) before or forgets to specify
first vertex(), proper error message is shown.
Merge pull request #2649 from JakubValtar/shape-fix

Corrected a typo in Tessellator#addQuadraticVertex()