Changeset b9fe89b for tests/concurrency

Make the symbol table's error-checking times explicit.

Previously, error checking happened on all WithSymbolTable? uses. Error checking means having a symbol-table add operation potentially cause a user-visible error report. Now, this only happens on the resolver pass's symbol table, while other passes' run in an "assert no errors can happen" mode.

An "ignore errors for now" mode is implemented too, which will be used in upcoming commits, for pre-resolver passes that use the symbol table.