enhancements in character constant parsing

* missing assertion converted to ErrorValue
* added test for 'a
* '\' must be error (KT-461)

compiler/testData/checkerWithErrorTypes/quick/IncorrectCharacterLiterals.jet

@@ -4,6 +4,9 @@
 fun ff() {
     val b = <!ERROR_COMPILE_TIME_VALUE!>''<!>
     val c = <!ERROR_COMPILE_TIME_VALUE!>'23'<!>
+    val d = <!ERROR_COMPILE_TIME_VALUE!>'a<!>
+    val e = <!ERROR_COMPILE_TIME_VALUE!>'ab<!>
+    val f = <!ERROR_COMPILE_TIME_VALUE!>'\'<!>
 }
 
 fun test() {
@@ -20,4 +23,4 @@ fun test() {
     <!ERROR_COMPILE_TIME_VALUE!>'\123'<!>
     <!ERROR_COMPILE_TIME_VALUE!>'\ra'<!>
     <!ERROR_COMPILE_TIME_VALUE!>'\000'<!>
-}
+}