Added an error for missing data in DEM.

src/main/java/org/orekit/rugged/errors/RuggedMessages.java

...resources/assets/org/orekit/rugged/RuggedMessages_de.utf8

...resources/assets/org/orekit/rugged/RuggedMessages_en.utf8

...resources/assets/org/orekit/rugged/RuggedMessages_es.utf8

...resources/assets/org/orekit/rugged/RuggedMessages_fr.utf8

...resources/assets/org/orekit/rugged/RuggedMessages_gl.utf8

...resources/assets/org/orekit/rugged/RuggedMessages_it.utf8

...resources/assets/org/orekit/rugged/RuggedMessages_no.utf8

...resources/assets/org/orekit/rugged/RuggedMessages_ro.utf8

...est/java/org/orekit/rugged/errors/RuggedMessagesTest.java