Better frontend handling when API is down

[jferas]

Resolves #367

Made inability to connect to Umbra API a softer error so native tokens can be transferred

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[github-actions]

Coverage after merging better-handling-when-api-down into master will be

81.15%

Coverage Report

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
contracts-periphery/src
UmbraBatchSend.sol	100%	100%	100%	100%
UniswapWithdrawHook.sol	100%	100%	100%	100%
umbra-js/src
ethers.ts	88.24%	100%	78.57%	100%
types.ts	50%	100%	0%	100%
umbra-js/src/classes
KeyPair.ts	98.65%	96.49%	100%	100%	31–32
RandomNumber.ts	100%	100%	100%	100%
StealthKeyRegistry.ts	100%	100%	100%	100%
TxHistoryProvider.ts	77.55%	76.47%	66.67%	79.31%	11, 11, 19–20, 31, 38, 44, 53, 57, 8
Umbra.ts	75.21%	63.01%	82.93%	81.95%	103, 132–133, 154, 226–228, 312–315, 377, 391, 398–400, 402–403, 406–407, 425, 427, 429, 429, 429, 429–430, 430, 430–432, 447, 447, 447–448, 452, 455, 475, 493–494, 581, 598, 611–612, 622–623, 636, 639–640, 670–671, 747, 751, 751, 751–754, 754, 754, 754, 754, 754, 754–755, 755, 755, 755, 755, 755, 755, 755, 755, 755, 755, 757, 757, 757–758, 761, 761, 761, 761, 761–762, 765, 774, 774, 783, 786, 786, 786–788, 801, 805, 808–809, 815–817, 824, 824, 824–827, 858, 863–864, 864–865
umbra-js/src/typechain
index.ts	70.59%	100%	37.50%	100%
umbra-js/src/typechain/factories
index.ts	100%	100%	100%	100%
umbra-js/src/typechain/factories/@OpenZeppelin
index.ts	100%	100%	100%	100%
umbra-js/src/typechain/factories/@openzeppelin/contracts
index.ts	100%	100%	100%	100%
umbra-js/src/typechain/factories/@openzeppelin/contracts/access
Ownable__factory.ts	50%	100%	0%	66.67%	70, 76
index.ts	50%	100%	0%	100%
umbra-js/src/typechain/factories/@openzeppelin/contracts/token
index.ts	100%	100%	100%	100%
umbra-js/src/typechain/factories/@openzeppelin/contracts/token/ERC20
ERC20__factory.ts	29.03%	0%	12.50%	47.06%	310, 314, 314, 314–315, 317, 326, 326, 326, 333, 333, 333, 336, 339, 345
IERC20__factory.ts	50%	100%	0%	66.67%	201, 204
index.ts	50%	100%	0%	100%
umbra-js/src/typechain/factories/contracts
IUmbraHookReceiver__factory.ts	50%	100%	0%	66.67%	61, 67
MockHook__factory.ts	22.58%	0%	0%	41.18%	107, 111, 111, 111–112, 114, 121, 121, 121, 126, 126, 126, 129, 132, 138, 144
StealthKeyRegistry__factory.ts	22.58%	0%	0%	41.18%	202, 206, 206, 206–207, 209, 216, 216, 216, 221, 221, 221, 224, 227, 233, 239
TestToken__factory.ts	64.52%	66.67%	50%	70.59%	329–330, 348, 351, 354, 360, 366
Umbra__factory.ts	70.97%	66.67%	62.50%	76.47%	495–496, 525, 529, 532, 538
index.ts	50%	100%	0%	100%
umbra-js/src/utils
cns.ts	50%	33.33%	50%	55%	33–36, 40, 40, 40, 40, 40–41, 44–46
constants.ts	100%	100%	100%	100%
ens.ts	36.11%	25%	33.33%	40%	25–27, 35, 56–57, 57, 57–58, 63–64, 66, 70, 70, 70, 70, 70–71, 75–77
utils.ts	76.67%	68.66%	76.92%	82%	109–111, 113–115, 121, 139–140, 169, 190, 246, 253–260, 260, 260, 260, 260, 260, 260–261, 263, 294, 296–297, 306, 306, 306–307, 309, 312, 321–322, 363, 380, 380, 380, 400, 429, 431, 436, 448, 448–449, 454, 456–457, 468–472, 478, 480, 522–523, 523, 523–525, 525, 525, 536, 59–60, 779–780, 792–793, 85, 93, 93
umbra-js/test
testPrivateKeys.ts	100%	100%	100%	100%
utils.ts	100%	100%	100%	100%

[garyghayrat]

lgtm!