More helpful errors

[marcua]

• All AybErrors are of one type with a message, which makes it hard to handle them programatically. Consider making each error have its own type and/or use an enum for them.
• Referencing the wrong entity results in Error: RowNotFound

$:~/Downloads/ayb$ ayb client register newcua
Error: Entity already exists
$:~/Downloads/ayb$ ayb client create_database marcua/test.sqlite
Error: Entity not found: "marcua"
$:~/Downloads/ayb$ ayb client create_database newcua/test.sqlite
Successfully created newcua/test.sqlite
$:~/Downloads/ayb$ ayb client query marcua/test.sqlite "CREATE TABLE favorite_databases(name varchar, score integer);"
Error: RowNotFound
$:~/Downloads/ayb$ ayb client query newcua/test.sqlite "CREATE TABLE favorite_databases(name varchar, score integer);"

Rows: 0

• From implementation in https://github.com/marcua/ayb/blob/main/src/error.rs translates the text of the errors, but doesn't show the example. e.g., AddressError says an email address was parsed wrong, but was it the to/from/reply_to? What was the actual email that wasn't valid?
• Better utilize actix's error handling functionality with typed AybErrors to modify return types (e.g., return 403/Forbidden on permissions errors, etc.)