Clean-up wording in various error messages (#103372)

The EmptyContent component is used to display full screen "empty"
messages - think errors, prompts to add content, 404s etc. They are
often paired with old cartoony illustrations, which isn't the look we're
going for right now.

The EmptyContent component does still retain support for displaying an
image though - sometimes it is genuinely useful, like when displaying
theme screenshots.

This removes cutsey wording like 'uh oh' from error messages using the
<EmptyContent> component. It also switches to use the brand font for
the title.

Author: dsas

client/assets/images/difm/site-build-in-progress.svg

@@ -40,7 +40,6 @@ class ConversationsEmptyContent extends Component {
 				) }
 				action={ action }
 				secondaryAction={ secondaryAction }
-				illustration=""
 			/>
 		);
 	}

client/assets/images/difm/website-content-required.svg

@@ -30,7 +30,6 @@ import PropTypes from 'prop-types';
 import { stringify, parse } from 'qs';
 import { Component } from 'react';
 import { connect } from 'react-redux';
-import Illustration from 'calypso/assets/images/domains/domain.svg';
 import DomainSearchResults from 'calypso/components/domains/domain-search-results';
 import ExampleDomainSuggestions from 'calypso/components/domains/example-domain-suggestions';
 import FreeDomainExplainer from 'calypso/components/domains/free-domain-explainer';
@@ -751,12 +750,7 @@ class RegisterDomainStep extends Component {
 		return (
 			<>
 				{ this.renderBestNamesPrompt() }
-				<EmptyContent
-					title=""
-					className="register-domain-step__placeholder"
-					illustration={ Illustration }
-					illustrationWidth={ 280 }
-				/>
+				<EmptyContent title="" className="register-domain-step__placeholder" />
 			</>
 		);
 	}