Update exception terminology to clarify types of exceptions

[BahaaMohamed98]

pull request

Clarify classification of exceptions and errors in Java introduction

This PR:

• Distinguishes clearly between exceptions and errors
• Clarifies that errors are not exceptions
• Moves the section on errors into its own separate category

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[jagdish-15]

Hey @BahaaMohamed98,

Did you happen to discuss these changes on the Exercism forum before opening the PR?
If you’ve already created a topic, please share the link here. Otherwise, you should start a discussion there first and then proceed with the PR.

[BahaaMohamed98]

Hey @jagdish-15! You're right — I forgot to open a forum topic first 😅
I'll go ahead and create one now and link it here shortly. Thanks for the heads up!

[BahaaMohamed98]

Just created the discussion here: http://forum.exercism.org/t/clarifying-error-vs-exception-in-java-exception-concept/17914