Refactor error messages for missing variables.

[rasulov-emirlan]

Before we would return error right aways. Now when we encounter an error on parsing stage, we try to list all the missing or incorrect envs. This should make debugging for our users easier. Seeing all the missing and incorrect envs, will help us fix configs faster. Also added some tests for the new error type.

Here is an example of what error message might look like:

missing required environment variables: 
	"PORT"
	"JWT_SALT"
	"READ_TIMEOUT"
parsing errors for environment variables: 
	field WriteTimeout env "WRITE_TIMEOUT": time: invalid duration "incorrect"

We might even make the error type public in the future. So our users will have an ability to choose how the want to represent these errors.

PS: thank you for this amazing package