Add handling for missing case in Asset class

[Guite]

If a normal asset (non zasset) path does not exist in the public folder yet we run into a problem because this check returns false. Afterwards we have an undefined/wrong behaviour because then it proceeds with the bundle resolving sections although a normal path is given. As a result, this line results in Notice: Undefined offset: 1.
So the least we should do is throwing an exception in line 99 then. Or if possible copy the missing file (as a fallback) and return its path afterwards.

[craigh]

closed by #4392