cmd/compile/internal/syntax: fix error handling for Read* calls.

[griesemer]

Tracking issue.

Proposal:

1. If there's any error, we always return the first error, whether there is an errh installed or not.
2. If an errh is installed, we call it for each error.
3. If no errh is installed, we exit after the first error.

[gopherbot]

CL https://golang.org/cl/32950 mentions this issue.