[Tools]Throw exception if a solution reference a not exist csproj.

[wyunchi-ms]

Description

Problem want to solve

1. CI still pass when a referenced csproj file is removed. But this should be detected.

Solution

1. We will detect the existance of every csproj file referenced by the solution files and throw exception if any of them doesn't exist.

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
    • {Please put the link here}
  • the markdown help files have been regenerated using the commands listed here

[wyunchi-ms]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[wyunchi-ms]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[BethanyZhou]

LGTM