GitLab

Adds reporting of errors for the errors detected:

 * Missing starting moveto
 * Missing command verb
 * Unexpected input type (number, arc flag)

The parsing helper parseArcFlag() is adjusted to not consume any
character on error.

BUG=231612

Review URL: https://codereview.chromium.org/1642463004

Cr-Commit-Position: refs/heads/master@{#371849}