prevent UI from trying to load samples during rebalance (was: Loading sample db beer-sample during rebalance gives "Unexpected server error, request logged")

SUBJ. See below for details. IMHO simples possible way to deal with that is to prevent UI from trying that.

Loading sample db beer-sample during rebalance gives "Unexpected server error, request logged". The error message can be improved for user convenience.

The Log tab correctly shows the error:

Server error during processing: ["web request failed",

{path,"/sampleBuckets/install"}

,

{type,exit}

,
{what,
{{

{badmatch,rebalance_running}

,
[

{samples_loader_tasks, create_sample_bucket,1}

,

{samples_loader_tasks, start_new_loading_task,1}

,

{samples_loader_tasks,handle_call,3}

,

{gen_server,handle_msg,5}

,

{proc_lib,init_p_do_apply,3}]},
{gen_server,call,
[samples_loader_tasks, {start_loading_sample,"beer-sample"},
infinity]}}},
{trace,
[{gen_server,call,3}, {menelaus_web, '-handle_post_sample_buckets/1-lc$^0/1-0-', 1}, {menelaus_web,handle_post_sample_buckets,1}, {menelaus_web,loop,3}, {mochiweb_http,headers,5},{proc_lib,init_p_do_apply,3}

]}]

Steps to reproduce:
1. Load 50K items in default bucket.
2. Rebalance 1 -> 4 nodes.
3. While rebalance is in progress, load the sample beer-sample from the UI "Settings" -> "Sample Buckets". Attached error is returned.