rustbot should recover better from parsing errors

[asquared31415]

rustbot seems to be very particular about its parsing, and if anything fails, the whole command fails.

here @rustbot label -needs-triage +A-diagnostics +D-papercut + D-incorrect I attempted to relabel some things, but accidentally had a space between a + and the label name, and rustbot completely failed to apply any of the label changes. It would have been nice if it at least applied the labels that parsed correctly.

and here @rustbot claim I'll work on fixing the suggestion at least, and possibly see about clarifying the diagnostic I attempted to assign myself after some (short) thinking, but I didn't realize that @rustbot claim must not have anything after it. It would be nice if rustbot just ignored things after the expected end of command, assuming them to be extra comments.

[Mark-Simulacrum]

The goal of the current behavior is to make it less likely that a change in how we parse things (e.g., adding a new parameter) causes an existing command to parse differently.