Provide parser errors

[danyill]

Currently if the parser produces errors when opening a file they are not conveyed to the end-user.

For instance the attached file produces an error and only one IED exists because there is a missing namespace declaration.

XAT_BusA_P1.cid.zip

We should be alerting the end user to parser errors and provide details as most applications do this (and silent errors like this are quite confusing).

I believe if there are errors, a node, parsererror is created under the root node.

For the above file (in Chrome) this is:

<parsererror xmlns="http://www.w3.org/1999/xhtml" style="display: block; white-space: pre; border: 2px solid #c77; padding: 0 1em 0 1em; margin: 1em; background-color: #fdd; color: black"><h3>This page contains the following errors:</h3><div style="font-family:monospace;font-size:12px">error on line 157499 at column 206: Namespace prefix esld for x on IED is not defined
</div><h3>Below is a rendering of the page up to the first error.</h3></parsererror>

We could be retrieving this with:

const errorNode = doc.querySelector('parsererror');
if (errorNode) {
  errorText = errorNode.textContent
} else {
  // parsing succeeded
}

If we just use textContent we would get something like:

This page contains the following errors:error on line 157499 at column 206: Namespace prefix esld for x on IED is not defined\nBelow is a rendering of the page up to the first error.

Arguably if the file isn't valid XML we should just barf and refuse to open/process it but I think warning the end user would be better.

A more complete answer might be as described here but I think this might be excessive (?).