Fixed 500 on missing license files

[snipe]

This would only happen if uploaded license files are missing. This checks to see that the file exists before it tries to get stats on the file.

[what-the-diff]

• Added a check to see if the file exists before displaying it.
• If the file doesn't exist, we add tags around it so that users know they can no longer download this asset (since its been deleted).
• We also added checks for when files don't exist in order to prevent errors from being thrown by Laravel's Storage facade methods like size() and exists(). This is because those methods throw exceptions instead of returning false or null values on failure, which makes them difficult to use with Blade templates without adding lots of try/catch blocks everywhere you want to call one of these functions.