Consider revising 'Strings to Numbers' exercise in episode 03

Make it explicit that trying to recast types that don't make sense will throw an exception, with a demonstration.
Then include another nonsensical conversion for the actual exercise.

_episodes/03-types-conversion.md

@@ -246,7 +246,7 @@ first is 2 and second is 5
 
 > ## Strings to Numbers
 >
-> `float` will convert a string to a floating point number,
+> Where reasonable, `float` will convert a string to a floating point number,
 > and `int` will convert a floating point number to an integer:
 >
 > ~~~
@@ -261,7 +261,24 @@ first is 2 and second is 5
 > ~~~
 > {: .output}
 >
-> Given that,
+> If the conversion doesn't make sense, however, an error message will occur
+>
+> ~~~
+> print("string to float:", float("Hello world!"))
+> ~~~
+> {: .python}
+>
+> ~~~
+> ---------------------------------------------------------------------------
+> ValueError                                Traceback (most recent call last)
+> <ipython-input-5-df3b790bf0a2> in <module>()
+> ----> 1 print("string to float:", float("Hello world!"))
+>
+> ValueError: could not convert string to float: 'Hello world!'
+> ~~~
+> {: .error}
+>
+> Given this information,
 > what do you expect this program to do?
 > What does it actually do?
 > Why do you think it does that?