A Value error is

Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value

the float function can take a string, ie float('5'), it's just that the value 'string' in float('string') is an inappropriate (non-convertible) string

On the other hand,

Passing arguments of the wrong type (e.g. passing a list when an int is expected) should result in a TypeError

so you would get a TypeError if you tried float(['5']) because a list can never be converted into a float.

Cite

Python exceptions can be caught in this way:

try:
<your code>
except <Exception>:
    <CODE 2>

OR LIKE THIS

try:
    <your code>
except(<exception1>,<exception2>):
    <Code to handle exception>

You are simply handling multiple exceptions together. You can always split them. They are not 2 different ways. In your case the as is for logging it .

Here are a few examples:

try:
    <code>
except TypeError:
    <Code for handling exception>
except ValueError:
    <Code for handling exception>
except ValidationError:
    <Code for handling exception>
except:
    <Code for handling exception>

In the last case it catches exception of any type since no type is specified.
In Python programs can raise any exception for anything.
In fact exception is just a special class, even you can create one for your library.

So the best way to find about the exception is to read the docs of the library not the exception class.

If your program catches the exception and wants more detail about it for creating a log file the code can be written like this.

except TypeError as e:
    i=str(e)

In this case you are catching the exception and converting its detail to a string.
This is from the Django docs about the error which you are talking about.

Form validation happens when the data is cleaned. If you want to customize this process, there are various places to make changes, each one serving a different purpose. Three types of cleaning methods are run during form processing. These are normally executed when you call the is_valid() method on a form. There are other things that can also trigger cleaning and validation (accessing the errors attribute or calling full_clean() directly), but normally they won’t be needed.

In general, any cleaning method can raise ValidationError if there is a problem with the data it is processing, passing the relevant information to the ValidationError constructor. See below for the best practice in raising ValidationError. If no ValidationError is raised, the method should return the cleaned (normalized) data as a Python object.

Some further references:

The link to docs
This link has info about other common builtin exception classes.