cmd/compile/internal/syntax: provide BadExpr where needed, call correct error handler
- For "if" statements without a condition, provide a BadExpr rather than nil
(clients expect IfStmt.Cond != nil since the parser is taking care of
reporting a missing condition).
- For 3-index slice expressions, also provide BadExpr where an index is
required but missing.
- Declare a parser-local error method to hide the embedded error method
so we don't use it by mistake.
Accidentally found while adjusting prototype parser to work for generics.
Change-Id: Iacc211cc60869be05efe9ae630d65dff1dac00a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/261218
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com>