platform: fix panic() when trying to print an unset ImageFormat

[mvo5]

During the development of the image pipeline refactor I noticed that the platform code will panic for unset image types. This is a bug and I added a small test to ensure it won't happen again.

[0] #506

[supakeen]

Do you mean that a call to panic is considered a bug or the message not including any details?

[mvo5]

Do you mean that a call to panic is considered a bug or the message not including any details?

Sorry, What I mean is that if one does:

func TestImageFormatString(t *testing.T) {
	assert.Equal(t, "unset", platform.FORMAT_UNSET.String())
}

right now it will print:

go build -v && go test && go vet
--- FAIL: TestImageFormatString (0.00s)
panic: invalid image format [recovered]
	panic: invalid image format

instead of printing "unset". Or maybe this is desired (if it is I am not sure I like it :)

It did bite me in #506 where something else in the code did not set the ImageFormat in platform.BasePlatform and suddently a panic() happend when the code ran someting like: fmt.Sprintf("unsupported image format %s", platform.ImageFormat). I hope this makes some more sense now (sorry that I did not explain this better).

[supakeen]

Then I understand :)