Implement error handling for EPSG code parsing (#123)

Co-authored-by: Claude <[email protected]>

Author: asinghvi17

src/crs.jl

@@ -133,6 +133,9 @@ end
 
 function GFT.EPSG(crs::CRS)
     str = proj_get_id_code(crs)
+    if isnothing(str)
+        error("Could not parse $crs to EPSG code; it probably does not correspond to one.")
+    end
     code = parse(Int, str)
     return GFT.EPSG(code)
 end

test/libproj.jl

@@ -489,6 +489,10 @@ end
     @test GFT.EPSG(crs) == GFT.EPSG("EPSG:4326")
 
     @test convert(GFT.EPSG, crs) == GFT.EPSG("EPSG:4326")
+
+    # Test error handling when CRS doesn't have an EPSG code
+    crs_without_epsg = Proj.CRS("+proj=merc +lat_ts=56.5 +ellps=GRS80 +type=crs")
+    @test_throws r"Could not parse.*EPSG code"is GFT.EPSG(crs_without_epsg)
     @test convert(GFT.WellKnownText2, crs) == GFT.WellKnownText2(crs)
     @test Proj.proj_get_id_code(convert(Proj.CRS, GFT.EPSG("EPSG:4326"))) ==
           Proj.proj_get_id_code(crs)