doctool: propagate errors from inspectAllTastyFiles

it's not pretty, but it does work.

tested with:
```
sbt 'scaladoc/runMain dotty.tools.scaladoc.Main ./bad -d a -siteroot empty'
```
and it will produce something like
```
-- Error: src/main/scala/Main.scala:26:12 --------------------------------------
undefined: m.parse # -1: TermRef(TermRef(NoPrefix,val m),parse) at readTasty
1 error found
Failure
[error] nonzero exit code returned from runner: 1
[error] (scaladoc / Compile / runMain) nonzero exit code returned from runner: 1
```

Author: katrinafyi

scaladoc/src/dotty/tools/scaladoc/tasty/TastyParser.scala

@@ -160,7 +160,9 @@ object ScaladocTastyInspector:
       report.error("File extension is not `tasty` or `jar`: " + invalidPath)
 
     if tastyPaths.nonEmpty then
-      TastyInspector.inspectAllTastyFiles(tastyPaths, jarPaths, classpath)(inspector)
+      val tastyOk = TastyInspector.inspectAllTastyFiles(tastyPaths, jarPaths, classpath)(inspector)
+      if !tastyOk then
+        report.error("Failure")
 
     val all = inspector.topLevels.result()
     all.groupBy(_._1).map { case (pckName, members) =>