fix(from_str, try_from_into): custom error types

Remove the use of trait objects as errors from `from_str` and
`try_from_into`; they seem to have caused a lot of confusion in
practice. (Also, it's considered best practice to use custom error
types instead of boxed errors in library code.) Instead, use custom
error enums, and update hints accordingly. Hints also provide
some guidance about converting errors, which could be covered
more completely in a future advanced errors section.

Also move from_str to directly after the similar exercise `from_into`,
for the sake of familiarity when solving.

info.toml

@@ -925,6 +925,27 @@ mode = "test"
 hint = """
 Follow the steps provided right before the `From` implementation"""
 
+[[exercises]]
+name = "from_str"
+path = "exercises/conversions/from_str.rs"
+mode = "test"
+hint = """
+The implementation of FromStr should return an Ok with a Person object,
+or an Err with an error if the string is not valid.
+
+This is almost like the `from_into` exercise, but returning errors instead
+of falling back to a default value.
+
+Hint: Look at the test cases to see which error variants to return.
+
+Another hint: You can use the `map_err` method of `Result` with a function
+or a closure to wrap the error from `parse::<usize>`.
+
+Yet another hint: If you would like to propagate errors by using the `?`
+operator in your solution, you might want to look at
+https://doc.rust-lang.org/stable/rust-by-example/error/multiple_error_types/reenter_question_mark.html
+"""
+
 [[exercises]]
 name = "try_from_into"
 path = "exercises/conversions/try_from_into.rs"
@@ -933,17 +954,19 @@ hint = """
 Follow the steps provided right before the `TryFrom` implementation.
 You can also use the example at https://doc.rust-lang.org/std/convert/trait.TryFrom.html
 
-You might want to look back at the exercise errors5 (or its hints) to remind
-yourself about how `Box<dyn Error>` works.
+Hint: Is there an implementation of `TryFrom` in the standard library that
+can both do the required integer conversion and check the range of the input?
 
-If you're trying to return a string as an error, note that neither `str`
-nor `String` implements `error::Error`. However, there is an implementation
-of `From<&str>` for `Box<dyn Error>`. This means you can use `.into()` or
-the `?` operator to convert your string into the correct error type.
+Another hint: Look at the test cases to see which error variants to return.
 
-If you're having trouble with using the `?` operator to convert an error string,
-recall that `?` works to convert `Err(something)` into the appropriate error
-type for returning from the function."""
+Yet another hint: You can use the `map_err` or `or` methods of `Result` to
+convert errors.
+
+Yet another hint: If you would like to propagate errors by using the `?`
+operator in your solution, you might want to look at
+https://doc.rust-lang.org/stable/rust-by-example/error/multiple_error_types/reenter_question_mark.html
+
+Challenge: Can you make the `TryFrom` implementations generic over many integer types?"""
 
 [[exercises]]
 name = "as_ref_mut"
@@ -951,15 +974,3 @@ path = "exercises/conversions/as_ref_mut.rs"
 mode = "test"
 hint = """
 Add AsRef<str> as a trait bound to the functions."""
-
-[[exercises]]
-name = "from_str"
-path = "exercises/conversions/from_str.rs"
-mode = "test"
-hint = """
-The implementation of FromStr should return an Ok with a Person object,
-or an Err with an error if the string is not valid.
-This is almost like the `try_from_into` exercise.
-
-If you're having trouble with returning the correct error type, see the
-hints for try_from_into."""