fix(fs): FileSystem error handling, eliminate SystemUtilities

[justinmk3]

Note: This commit is part of a series. Continued from https://github.com/aws/aws-toolkit-vscode/pull/5267

Problem

• Error handling is inconsistent.
• SystemUtilities is redundant with fs.ts, and is not web-compatible.

Solution

• Merge the features of SystemUtilities into fs.ts.
• Migrate tests.

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[justinmk3]

/runintegrationtests