Mention that Array.front/back throw error if empty

(cherry picked from commit 4ff1a341717de6df6c4485d2f07c956656a253c8)

doc/classes/Array.xml

@@ -99,7 +99,7 @@
 			<return type="Variant">
 			</return>
 			<description>
-				Returns the last element of the array, or [code]null[/code] if the array is empty.
+				Returns the last element of the array. Throws an error and returns [code]null[/code] if the array is empty.
 			</description>
 		</method>
 		<method name="bsearch">
@@ -192,7 +192,7 @@
 			<return type="Variant">
 			</return>
 			<description>
-				Returns the first element of the array, or [code]null[/code] if the array is empty.
+				Returns the first element of the array. Throws an error and returns [code]null[/code] if the array is empty.
 			</description>
 		</method>
 		<method name="has">