Full error messages don't include the key name

[stind]

This issue was initially reported in discourse: https://discourse.dry-rb.org/t/inconsistency-in-resolving-full-error-messages/1091

1. Full message of a key failure does not include the key name when the key is not translated and the failure method was called with a symbol message, e.g. key.failure(:is_invalid).
   So the error message looks like " is invalid", instead of "foo is invalid".

2. Full message of a base failure starts with a space symbol, e.g. " the message".

To Reproduce

require "i18n"
require "dry/validation"

I18n.backend.store_translations(
  :en,
  {
    dry_validation: {
      errors: {
        is_invalid: "is invalid"
      }
    }
  }
)

class MyContract < Dry::Validation::Contract
  config.messages.backend = :i18n

  params do
    required(:foo).value(:integer)
  end

  rule do
    base.failure("base failure here")
  end

  rule(:foo) do
    key.failure(:is_invalid)
  end
end

MyContract.new.call(foo: 42).errors(full: true).map(&:text)
#=> [" base failure here", " is invalid"]
# Expected:
#=> ["base failure here", "foo is invalid"]

Expected behavior

1. Full message of a key failure always starts with the key name, even if the key is not translated
2. Full message of a base failure includes just the message without the leading space symbol, e.g. "the message" instead of " the message"

Environment

ruby: 2.6.6
dry-validation: 1.5.2