Fixed Issue 175: Command does not load dotenv files if some do not exist [1]

[egasimov]

Hi, The following PR is intended to resolve issue-175 .

When the any of the passed fileNames do not exist, it just silently stops the process of 'load env files'
and start to execute the command provided.

I believe that behaviour might lead to unexpected results and assumptions which might affect the correctness of the system that utilizes the environment variables using godotenv command
And decided to make small PR.

Current behaviour:
Silently stopping the process of 'load env files' and continuing to execute the command provided.

$ cat .env
FOO="Using .env"

$ godotenv -f ".env,.env.doesnotexist" env | grep FOO
FOO=Using .env

$ godotenv -f ".env.doesnotexist,.env" env | grep FOO
# FOO does not get set

Considered behaviour:
When the fileName do not exist, then print the message "The system cannot find the file spec" and exit.

$ cat .env
FOO="Using .env"

$ godotenv -f ".env,.env.doesnotexist" env | grep FOO
FOO=Using .env

$ godotenv -f ".env.doesnotexist,.env" env | grep FOO
open .env.doesnotexist: The system cannot find the file spec