Returning a String with line break from an Apex method (AuraEnabled)
Thanks to EricSSH's suggestion of escape="false", I did a quick search for any Aura tags that would escape HTML, and found https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/aura_compref_aura_unescapedHtml.htm
This did help me include a line break within a String with the use of '< br/>' and displaying it on the component as follows
<aura:unescapedHtml value="{!v.yourString}" />
I needed a solution that didn't require modifying the string and found this to work in Chrome and Firefox.
<pre>{!v.yourString}</pre>
The <pre/>
html tag diplays text in a fixed-width non-wrapped text, but the following CSS made it blend in with the rest of the page.
.THIS pre {
white-space: pre-wrap; /* Since CSS 2.1 */
white-space: -moz-pre-wrap; /* Mozilla, since 1999 */
white-space: -pre-wrap; /* Opera 4-6 */
white-space: -o-pre-wrap; /* Opera 7 */
word-wrap: break-word; /* Internet Explorer 5.5+ */
font-family: inherit !important;
}