docs(exercises): updated all exercises readme files

all exercises readme files now have a unified structure and a description

exercises/error_handling/README.md

@@ -1,11 +1,11 @@
-For this exercise check out the sections:
+# Error handling
+Most errors aren’t serious enough to require the program to stop entirely. 
+Sometimes, when a function fails, it’s for a reason that you can easily interpret and respond to.
+For example, if you try to open a file and that operation fails because the file doesn’t exist, you might want to create the file instead of terminating the process.
+
+## Further information
+
 - [Error Handling](https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html)
 - [Generics](https://doc.rust-lang.org/book/ch10-01-syntax.html)
-
-of the Rust Book.
-
-or alternatively, check out the sections:
 - [Result](https://doc.rust-lang.org/rust-by-example/error/result.html)
 - [Boxing errors](https://doc.rust-lang.org/rust-by-example/error/multiple_error_types/boxing_errors.html)
-  
-of the Rust By Example Book.