Valid date message when date is invalid (aces#8279)

When using /api/v0.0.3/candidates with an invalid date format, returns 400 Bad Requests/JSON errors.
This  checks the date of birth and returns the correct format in the message ('yyyy-mm-dd') and returns an appropriate error message if the check fails.

modules/api/php/endpoints/candidates.class.inc

@@ -241,7 +241,7 @@ class Candidates extends Endpoint implements \LORIS\Middleware\ETagCalculator
                 $pscid,
                 $project->getId()
             );
-        } catch (\LorisException $e) {
+        } catch (\LorisException | \InvalidArgumentException $e) {
             return new \LORIS\Http\Response\JSON\BadRequest($e->getMessage());
         }
 

php/libraries/Candidate.class.inc

@@ -18,6 +18,7 @@ define('PSCID_NOT_UNIQUE', 3);
 define('PSCID_INVALID_STRUCTURE', 4);
 define('EDC_NOT_SPECIFIED', 5);
 define('DOB_NOT_SPECIFIED', 6);
+define('DOB_INVALID', 7);
 
 /**
  * Wrapper around a candidate in Loris. Mostly, it gets information
@@ -273,15 +274,19 @@ class Candidate implements \LORIS\StudyEntities\AccessibleResource,
                 EDC_NOT_SPECIFIED
             );
         }
-        if ($dateOfBirth !== null
-            && (DateTime::createFromFormat('Y-m-d', $dateOfBirth) === false
-            || empty($dateOfBirth))
-        ) {
+        if (empty($dateOfBirth)) {
             throw new InvalidArgumentException(
                 "Date of Birth must be specified",
                 DOB_NOT_SPECIFIED
             );
         }
+        $dob = DateTime::createFromFormat('!Y-m-d', $dateOfBirth);
+        if ($dob === false) {
+            throw new InvalidArgumentException(
+                "Date of Birth is invalid (expected format: YYYY-MM-DD)",
+                DOB_INVALID
+            );
+        }
 
         if ($PSCIDSettings['generation'] == 'user') {
             // check pscid is specified