Add compile warning if player sprite sheet isn't set

[pau-tomas]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Add compile warning if player sprite sheet is set to None.

• What is the current behavior? (You can also link to an open issue here)

Compile fails with TypeError: Cannot read property 'plugin' of undefined when no sprite sheet has been selected for the Player. See #447

• What is the new behavior (if this is a feature change)?

A more clear Player Sprite Sheet isn't set. Please, make sure to select a Sprite Sheet in the Project editor. error is shown on compile.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

• Other information:

This doesn't solve the core issue of #447 (how did the player sprite sheet ended up being None?). But it should help users to fix the issue by themselves if they get there.

[chrismaltby]

Thanks @pau-tomas I can reproduce the error by just deleting the currently active player sprite sheet png file and building. This error is much better than the random plugin one that appears