better error handling when custom sobject is not found

[nimajalali]

Today if you attempt to interact with an SObject that doesn't exist in SalesForce the force package will panic.

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x48 pc=0x49f87]

goroutine 16 [running]:
runtime.panic(0x2a48a0, 0x4638e4)
    /usr/local/Cellar/go/1.3.3/libexec/src/pkg/runtime/panic.c:279 +0xf5
github.com/nimajalali/go-force/force.(*ForceApi).GetSObject(0xc208004180, 0x2eba50, 0xe, 0x51def0, 0xc208210000, 0x0, 0x0)
    /Users/njalali/Documents/go/src/github.com/nimajalali/go-force/force/sobjects.go:68 +0x137

Instead of a panic we should return a meaningful error. To do this we should verify the object metadata exists in the forceApi.apiSObjects map before trying to access it.

e.g.

data, ok := forceApi.apiSObjects["SOME_OBJECT_NAME"]
if !ok {
   return fmt.Errorf("Unable to find metadata for object: %s", "SOME_OBJECT_NAME")
}