Fix to load dependent assembly from specified path

[leyarx]

Purpose

Small fix to enable loading dependent assemblies from specified path

Description

Fixed ArgumentOutOfRangeException from .Substring() by checking length parameter
- in case when we are running .IndexOf(',') on already stripped from commas "NameOfYourAddInDll"
Added check to skip searching for the assembly if assembly with specified name is already loaded
- in case when AssemblyResolve tries to find "NameOfYourAddInDll.resources" and ".resources" is stripped from the name
Fixed return value from ShowDialog() to return true after closing with Ok button
Removed confusing (duplicated) ShowWarning() when the dependent assembly can't be loaded

Declarations

Check these if you believe they are true

• This PR fix bug
• This PR for new feature
• The codebase is in a better state after this PR
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• Snapshot of UI changes, if any.
• This PR modifies some build requirements and the readme is updated

Reviewers

@chuongmep

FYIs

[chuongmep]

@leyarx Good catch ! Thank you so much for contribute.