Use user-error instead of error where appropriate.

[haji-ali]

This PR replace calls of (error ...) by the more approriate (user-error ...) where relevent.

According to the docs:

[...] a user error (or "pilot error") comes
from an incorrect manipulation by the user, not from an actual problem.
In contrast with other errors, user errors normally do not cause
entry to the debugger, even when debug-on-error is non-nil.

The old (error ...) calls would trip the debugger for normal user behavior (like scrolling beyond page 0 using the mouse-wheel).