Fix initial send failures being ignored if fallback is disabled

[brholstein]

Fixes #210

The diff is quite large but when you filter out the whitespace changes it is quite simple:

diff --git a/includes/wp-mail-api.php b/includes/wp-mail-api.php
index 2d910a1..4cf1487 100644
--- a/includes/wp-mail-api.php
+++ b/includes/wp-mail-api.php
@@ -495,8 +495,11 @@ function wp_mail( $to, $subject, $message, $headers = '', $attachments = array()
         }

         // Email Fallback
+        if (!$isFallbackNeeded) return true;
+
         $isFallbackEnabled = get_option('email_fallback') ?: 'no';
-        if ($isFallbackNeeded && $isFallbackEnabled === 'yes') {
+        if (!($isFallbackEnabled === 'yes')) return false;
+
         global $phpmailer;

         // (Re)create it, if it's gone missing.
@@ -735,9 +738,6 @@ function wp_mail( $to, $subject, $message, $headers = '', $attachments = array()
             return false;
         }
     }
-
-        return true;
-    }
 }

The if ($isFallbackNeeded && $isFallbackEnabled === 'yes') { check is not exhaustive of all the possibilities so when a fallback is needed, but not enabled it falls through to the default return value of true at the bottom of the function which is incorrect. To address this I've:

1. Added a guard statement to exit early with true it no fallback is needed.
2. Added a guard statement to exit early with false if a fallback is needed but disabled.

I've also removed the fall through return true at the end of the function as it is now unreachable.