Process xcrun failures when a local Xcode is used

The K/N compiler uses a toolchain and command line tools provided
by Xcode. But if Xcode or command line are not properly installed
the compiler fails with a cryptic error message (e.g. see #3587).

This patch processes failures during an xcrun execution and reports
a better error message.

shared/src/main/kotlin/org/jetbrains/kotlin/konan/Exceptions.kt

@@ -22,7 +22,12 @@ package org.jetbrains.kotlin.konan
 open class KonanException(message: String = "", cause: Throwable? = null) : Exception(message, cause)
 
 /**
- * An error occured during external tool invocation. Such as non-zero exit code. 
+ * An error occurred during external tool invocation. Such as non-zero exit code.
  */
 class KonanExternalToolFailure(message: String, val toolName: String, cause: Throwable? = null) : KonanException(message, cause)
 
+/**
+ * An exception indicating a failed attempt to access some parts of Xcode (e.g. get SDK paths or version).
+ */
+class MissingXcodeException(message: String, cause: Throwable? = null) : KonanException(message, cause)
+

shared/src/main/kotlin/org/jetbrains/kotlin/konan/target/Xcode.kt

@@ -16,6 +16,8 @@
 
 package org.jetbrains.kotlin.konan.target
 
+import org.jetbrains.kotlin.konan.KonanExternalToolFailure
+import org.jetbrains.kotlin.konan.MissingXcodeException
 import org.jetbrains.kotlin.konan.exec.Command
 import org.jetbrains.kotlin.konan.file.File
 
@@ -69,8 +71,11 @@ private object CurrentXcode : Xcode {
                 .removePrefix("Xcode ")
     }
 
-    private fun xcrun(vararg args: String): String =
+    private fun xcrun(vararg args: String): String = try {
-            Command("/usr/bin/xcrun", *args).getOutputLines().first() // TODO: handle execution error
+            Command("/usr/bin/xcrun", *args).getOutputLines().first()
+        } catch(e: KonanExternalToolFailure) {
+            throw MissingXcodeException("An error occurred during an xcrun execution. Make sure that Xcode and its command line tools are properly installed.", e)
+        }
 
     private fun getSdkPath(sdk: String) = xcrun("--sdk",  sdk, "--show-sdk-path")
 }