Standardizing Exception Message Style in Ruby

While reviewing dozens of PRs from ericproulx optimizing Grape’s internals recently, I noticed that the bare raise ArgumentError, "..." calls scattered across the codebase were inconsistent: some messages were capitalized, some ended in a period, most were not. Longtime readers know where this is going. Ruby’s own core and standard library exceptions don’t do this - TypeError: no implicit conversion from nil to integer, ArgumentError: wrong number of arguments, and so on all read lowercase and unpunctuated, because the message is meant to be read after the exception class name and a colon, not as a standalone sentence. Grape’s own Grape::Exceptions::* classes already follow this convention. The bare raise ArgumentError, "..." calls in dsl/entity.rb, dsl/inside_route.rb, dsl/validations.rb, and validations/types/dry_type_coercer.rb didn’t, so I fixed those seven sites and documented the convention in CONTRIBUTING.md, in #2909. To avoid regressions, I wrote rubocop-exception_messages, a…