Better handling for lexing errors.

test/comments/unterminated_block.wren

@@ -0,0 +1,3 @@
+// expect error line 3
+IO.write("nope") /*
+oops

test/comments/unterminated_nested_block.wren

@@ -0,0 +1,4 @@
+// expect error line 4
+IO.write("nope") /* /* /*
+*/
+oops

test/unexpected_character.wren

@@ -0,0 +1,3 @@
+IO.write("no")
+// Something that's not a valid character:
+^ // expect error