Build.ps1: More detailed exception printing

[tristanlabelle]

This should help diagnose errors such as in https://ci-external.swift.org/job/swift-main-windows-toolchain/34/console :

C:\Users\swift-ci\jenkins\workspace\swift-main-windows-toolchain\swift\utils\build.ps1 : Exception calling 
"DownloadFile" with "2" argument(s): "An exception occurred during a WebClient request."
    + CategoryInfo          : NotSpecified: (:) [build.ps1], MethodInvocationException
    + FullyQualifiedErrorId : WebException,build.ps1

Here's an example output from a synthetic DownloadFile failure:

[compnerd]

@swift-ci please smoke test

[compnerd]

CC: @shahmishal

[compnerd]

@swift-ci please smoke test Linux platform

[compnerd]

@swift-ci please smoke test macOS platform

[shahmishal]

Thanks I just merged it as it only impacts Windows CI.