Better handling of missing HGT data import error

[pmffromspace]

When importing a gps track which partially goes trough an area where no hgt files exist (example sea) the gps tracks failes to import with error:

java.io.FileNotFoundException: /maps/HGT/N56E017.hgt
	at com.sunlocator.topolibrary.HGTFileLoader_LocalStorage.loadHGT(HGTFileLoader_LocalStorage.java:24) ~[TopoLibrary-2.2-SNAPSHOT.jar!/:na]
	at com.sunlocator.topolibrary.HGTWorker.loadHGTFile_3DEM(HGTWorker.java:115) ~[TopoLibrary-2.2-SNAPSHOT.jar!/:na]
	at com.sunlocator.topolibrary.GPX.GPXWorker.getElevationDataFromHGT(GPXWorker.java:545) ~[TopoLibrary-2.2-SNAPSHOT.jar!/:na]
	at com.sunlocator.topolibrary.GPX.GPXWorker.replaceElevationData(GPXWorker.java:491) ~[TopoLibrary-2.2-SNAPSHOT.jar!/:na]

Altough this is somewhat expected behaviour, it is kind of not solveable because the file N56E017.hgt can not be downloaded because it does not exist.

So could there be some sort of default value for areas without hgt files (could be elevation 0 meter because its just sea) so realy long sailing trips for example can be imported and displayed.

My temporary workaround:

• use a hgt file from a peace of ocean which exists
• copy and rename it to the name of the not existent file
• gps tracks now can be imported and displayed