fix(mint): return raw connect errors, not strings

[arrangabriel]

Return Mint connect failure reason, not a formatted string

Currently, connect/2 inspects whatever ConnectionProcess.start_link/4 returns into a message, so a refused connection results in: {:error, "Error while opening connection: {:error, %Mint.TransportError{reason: :econnrefused}}"}. This is hard for callers to use, e.g. in a case expression. The Gun adapter returns transport errors directly.

Changes

• connect/2 and its catch :exit clause return the error tuple directly.

[sleipnir]

@arrangabriel you can take a look on failures?

Error:      test/grpc/integration/stub_test.exs:96
     Assertion failed, no matching message after 100ms
     The process mailbox is empty.
     code: assert_receive {:initial_call_succeeded, initial_channel, "Hello, first caller"}
     stacktrace:
       test/grpc/integration/stub_test.exs:109: anonymous fn/1 in GRPC.Integration.StubTest."test named Gun connections survive the original caller exiting"/1
       (grpc 1.0.5) test/support/integration_data_case.ex:47: GRPC.Integration.TestCase.run_server/4
       test/grpc/integration/stub_test.exs:97: (test)