Add better error messages to insertion marker (#4791)
Add better error messages to insertion marker (#4791)


google/blockly,mark-friedman/blockly,rachel-fenichel/blockly,google/blockly,mark-friedman/blockly,mark-friedman/blockly,mark-friedman/blockly,mark-friedman/blockly,rachel-fenichel/blockly,google/blockly,google/blockly,rachel-fenichel/blockly,google/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,google/blockly,rachel-fenichel/blockly,google/blockly,rachel-fenichel/blockly,mark-friedman/blockly