* convert pubGet to throw ToolExit on non-zero exit code
* convert commandValidator to throw ToolExit for non-zero exit code
* convert flutter commands to throw ToolExit for non-zero exit code
* use convenience method throwToolExit
* only show "if this problem persists" for unusual exceptions
This is causing issues when integratting Flutter into GN as the
generated depfile refers to snapshot as the target instead of the
bundle. We instead use a separate GN action to generate the
snapshot use the Flutter compiler to only assemble the bundle.
