Fix fixes in backend and improve warning message

In fact the warning occurrences in the backend were also assuming an empty
catch handler. Change them to do the right thing, and fix the warning message
to point out this possibility.

Author: odersky

compiler/src/dotty/tools/backend/jvm/BytecodeWriters.scala

@@ -117,8 +117,9 @@ trait BytecodeWriters {
       catch case ex: ClosedByInterruptException =>
         try
           outfile.delete() // don't leave an empty or half-written classfile around after an interrupt
-        catch case _: Throwable =>
-          throw ex
+        catch
+          case _: Throwable =>
+        throw ex
       finally outstream.close()
       report.informProgress("wrote '" + label + "' to " + outfile)
     }

compiler/src/dotty/tools/backend/jvm/GenBCode.scala

@@ -275,8 +275,9 @@ class GenBCodePipeline(val int: DottyBackendInterface, val primitives: DottyPrim
             catch case ex: ClosedByInterruptException =>
               try
                 outTastyFile.delete() // don't leave an empty or half-written tastyfile around after an interrupt
-              catch case _: Throwable =>
-                throw ex
+              catch
+                case _: Throwable =>
+              throw ex
             finally outstream.close()
 
             val uuid = new TastyHeaderUnpickler(binary()).readHeader()

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2622,8 +2622,10 @@ object Parsers {
       CaseDef(pat, grd, atSpan(accept(ARROW)) {
         if exprOnly then
           if in.indentSyntax && in.isAfterLineEnd && in.token != INDENT then
-            warning(i"""misleading indentation: this expression forms part of the preceding catch case,
-                       |it should be indented for clarity.""")
+            warning(i"""Misleading indentation: this expression forms part of the preceding catch case.
+                       |If this is intended, it should be indented for clarity.
+                       |Otherwise, if the handler is intended to be empty, use a multi-line catch with
+                       |an indented case.""")
           expr()
         else block()
       })