fix unhandled error occurs when loading a file with invalid XML forma…

[pontaGC]

Hello,

I discovered that an unexpected error occurs when reading an empty file as the schema file in the schema dialog.
The cause of the occurrence was that XmlException was not taken into account when loading the schema file.
This is a minor improvement, as currently there is error checking when committing changes in the schema dialog.

I tested loading the invalid schema file below and verified that no unexpected errors occur (I verified that it goes through the exception path I added).

1. empty file
2. XSD file with unclosed XML tag
3. XSD file opened in another process

[pontaGC]

@microsoft-github-policy-service agree