Validate zip file param validation for AWS Lambda

[jamesls]

This improves the error message when the customer does not provide a fileb:// prefix for the zip file argument. Now you get:

# Omitting any file/fileb prefix:
$ aws lambda update-function-code --function-name helloworld --zip-file tests.zip

--zip-file must be a file with the fileb:// prefix.
Example usage:  --zip-file fileb://path/to/file.zip

# Using file:// instead of fileb://

$ aws lambda update-function-code --function-name helloworld --zip-file file://tests.zip

Error parsing parameter '--zip-file': Unable to load paramfile (tests.zip), text contents could not be decoded.  If this is a binary file, please use the fileb:// prefix instead of the file:// prefix.

The same logic is applied to the create-function operation as well.

cc @kyleknap

[coveralls]

Coverage increased (+0.04%) to 92.89% when pulling 5dbc95b on jamesls:validate-zip-file into c910c87 on aws:develop.

[coveralls]

Coverage decreased (-0.11%) to 92.75% when pulling 86795d0 on jamesls:validate-zip-file into c910c87 on aws:develop.

[coveralls]

Coverage increased (+0.03%) to 92.88% when pulling 86795d0 on jamesls:validate-zip-file into c910c87 on aws:develop.

[kyleknap]

Looks good. 🚢