core, miner, xeth: renamed gas methods

core, ethdb, trie: validate database errors

* BuyGas => SubGas
* RefundGas => AddGas
* SetGasPool => SetGasLimit

mist: R.I.P.

     /"""""/""""""".
    /     /         \             __
   /     /           \            ||
  /____ /             \           ||
 |     |  In Loving    |          ||
 |     |   Memory      |          ||
 |     |               |          ||
 |     |   2014-2015   |          ||
 |     |     * *   * * |         _||_
 |     |     *\/* *\/* |        | TT |
 |     |     *_\_  /   ...""""""| || |.""...."""""""".""
 |     |         \/.."""""..."""\ || /.""".......""""...
 |     |...."""""""........""""""^^^^"......."""""""".."
 |......"""""""""""""""........"""""...."""""..""-Jeff W.

ethtest improvements

eth/fetcher: lower max cache size, add timeout slack

unit test coverage for NewDataArgs

Disassociates hardcoded tests to skip when running via CLI. Tests still
skipped when running `go test`

eth: fix the propagation/announce order for mined blocks