Surface stderr from failing proxy command

Some users are reporting trouble connecting to their workspaces, and receive an error "Server closed connection during identification exchange" on the "Choose IDE and project for workspace ... " screen.

Stack trace is:

```
java.lang.Throwable: Failed to retrieve IDEs (attempt 1; will retry in 5000 ms)
at com.intellij.openapi.diagnostic.Logger.error(Logger.java:370)
at com.coder.gateway.views.steps.CoderLocateRemoteProjectStepView$onInit$1$ides$2.invoke(CoderLocateRemoteProjectStepView.kt:212)
at com.coder.gateway.views.steps.CoderLocateRemoteProjectStepView$onInit$1$ides$2.invoke(CoderLocateRemoteProjectStepView.kt:186)
at com.coder.gateway.sdk.RetryKt.suspendingRetryWithExponentialBackOff(Retry.kt:63)
at com.coder.gateway.sdk.RetryKt$suspendingRetryWithExponentialBackOff$1.invokeSuspend(Retry.kt)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:104)
at com.intellij.openapi.application.impl.DispatchedRunnable.run(DispatchedRunnable.kt:43)
at com.intellij.openapi.application.TransactionGuardImpl.runWithWritingAllowed(TransactionGuardImpl.java:208)
at com.intellij.openapi.application.TransactionGuardImpl.access$100(TransactionGuardImpl.java:21)
at com.intellij.openapi.application.TransactionGuardImpl$1.run(TransactionGuardImpl.java:190)
at com.intellij.openapi.application.impl.ApplicationImpl.runIntendedWriteActionOnCurrentThread(ApplicationImpl.java:861)
at com.intellij.openapi.application.impl.ApplicationImpl$4.run(ApplicationImpl.java:478)
at com.intellij.openapi.application.impl.FlushQueue.doRun(FlushQueue.java:79)
at com.intellij.openapi.application.impl.FlushQueue.runNextEvent(FlushQueue.java:121)
at com.intellij.openapi.application.impl.FlushQueue.flushNow(FlushQueue.java:41)
at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:318)
at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:792)
at java.desktop/java.awt.EventQueue$3.run(EventQueue.java:739)
at java.desktop/java.awt.EventQueue$3.run(EventQueue.java:733)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:399)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:761)
at com.intellij.ide.IdeEventQueue.defaultDispatchEvent(IdeEventQueue.kt:690)
at com.intellij.ide.IdeEventQueue._dispatchEvent$lambda$10(IdeEventQueue.kt:593)
at com.intellij.openapi.application.impl.ApplicationImpl.runWithoutImplicitRead(ApplicationImpl.java:1485)
at com.intellij.ide.IdeEventQueue._dispatchEvent(IdeEventQueue.kt:593)
at com.intellij.ide.IdeEventQueue.access$_dispatchEvent(IdeEventQueue.kt:67)
at com.intellij.ide.IdeEventQueue$dispatchEvent$processEventRunnable$1$1$1.compute(IdeEventQueue.kt:369)
at com.intellij.ide.IdeEventQueue$dispatchEvent$processEventRunnable$1$1$1.compute(IdeEventQueue.kt:368)
at com.intellij.openapi.progress.impl.CoreProgressManager.computePrioritized(CoreProgressManager.java:787)
at com.intellij.ide.IdeEventQueue$dispatchEvent$processEventRunnable$1$1.invoke(IdeEventQueue.kt:368)
at com.intellij.ide.IdeEventQueue$dispatchEvent$processEventRunnable$1$1.invoke(IdeEventQueue.kt:363)
at com.intellij.ide.IdeEventQueueKt.performActivity$lambda$1(IdeEventQueue.kt:997)
at com.intellij.openapi.application.TransactionGuardImpl.performActivity(TransactionGuardImpl.java:105)
at com.intellij.ide.IdeEventQueueKt.performActivity(IdeEventQueue.kt:997)
at com.intellij.ide.IdeEventQueue.dispatchEvent$lambda$7(IdeEventQueue.kt:363)
at com.intellij.openapi.application.impl.ApplicationImpl.runIntendedWriteActionOnCurrentThread(ApplicationImpl.java:861)
at com.intellij.ide.IdeEventQueue.dispatchEvent(IdeEventQueue.kt:405)
at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:207)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:92)
```