some cleanup, BASIC line finding now properly uses stack

src/basic/engine/do_error.s

@@ -196,4 +196,4 @@ do_basic_error:                        // error code in .X
 	ldx #$FE
 	txs
 
-	jmp basic_main_loop
+	jmp shell_main_loop

src/basic/engine/execute_line_statements.s

@@ -164,7 +164,7 @@ execute_statements_end_of_line:
 
 	ldx CURLIN+1
 	inx
-	beq_16 basic_main_loop
+	beq_16 shell_main_loop
 
 	// Advance to the next line - first copy the line number
 	// to previous line number
@@ -181,7 +181,7 @@ execute_statements_end_of_line:
 
 	lda OLDTXT+0
 	ora OLDTXT+1
-	beq_16 basic_main_loop
+	beq_16 shell_main_loop
 	
 	// FALLTROUGH
 
@@ -191,7 +191,7 @@ execute_line:
 
 	ldy #0
 	jsr peek_line_pointer_null_check
-	bcc_16 basic_main_loop             // branch if end of program reached
+	bcc_16 shell_main_loop             // branch if end of program reached
 
 	// Skip pointer and line number to get address of first statement