Improve handling of Perl's block eval errors

Taken from `Perl::Critic`:

A common idiom in perl for dealing with possible errors is to use `eval`
followed by a check of `$@`/`$EVAL_ERROR`:

    eval {
        ...
    };
    if ($EVAL_ERROR) {
        ...
    }

There's a problem with this: the value of `$EVAL_ERROR` (`$@`) can change
between the end of the `eval` and the `if` statement. The issue are object
destructors:

    package Foo;

    ...

    sub DESTROY {
        ...
        eval { ... };
        ...
    }

    package main;

    eval {
        my $foo = Foo->new();
        ...
    };
    if ($EVAL_ERROR) {
        ...
    }

Assuming there are no other references to `$foo` created, when the
`eval` block in `main` is exited, `Foo::DESTROY()` will be invoked,
regardless of whether the `eval` finished normally or not. If the `eval`
in `main` fails, but the `eval` in `Foo::DESTROY()` succeeds, then
`$EVAL_ERROR` will be empty by the time that the `if` is executed.
Additional issues arise if you depend upon the exact contents of
`$EVAL_ERROR` and both `eval`s fail, because the messages from both will
be concatenated.

Even if there isn't an `eval` directly in the `DESTROY()` method code,
it may invoke code that does use `eval` or otherwise affects
`$EVAL_ERROR`.

The solution is to ensure that, upon normal exit, an `eval` returns a
true value and to test that value:

    # Constructors are no problem.
    my $object = eval { Class->new() };

    # To cover the possiblity that an operation may correctly return a
    # false value, end the block with "1":
    if ( eval { something(); 1 } ) {
        ...
    }

    eval {
        ...
        1;
    }
        or do {
            # Error handling here
        };

Unfortunately, you can't use the `defined` function to test the result;
`eval` returns an empty string on failure.

Various modules have been written to take some of the pain out of
properly localizing and checking `$@`/`$EVAL_ERROR`. For example:

    use Try::Tiny;
    try {
        ...
    } catch {
        # Error handling here;
        # The exception is in $_/$ARG, not $@/$EVAL_ERROR.
    };  # Note semicolon.

"But we don't use DESTROY() anywhere in our code!" you say. That may be
the case, but do any of the third-party modules you use have them? What
about any you may use in the future or updated versions of the ones you
already use?

src/script/hydra-notify

@@ -29,9 +29,11 @@ sub buildStarted {
         or die "build $buildId does not exist\n";
 
     foreach my $plugin (@plugins) {
-        eval { $plugin->buildStarted($build); };
-        if ($@) {
-            print STDERR "$plugin->buildStarted: $@\n";
+        eval {
+            $plugin->buildStarted($build);
+            1;
+        } or do {
+            print STDERR "error with $plugin->buildStarted: $@\n";
         }
     }
 }
@@ -53,9 +55,11 @@ sub buildFinished {
     }
 
     foreach my $plugin (@plugins) {
-        eval { $plugin->buildFinished($build, [@dependents]); };
-        if ($@) {
-            print STDERR "$plugin->buildFinished: $@\n";
+        eval {
+            $plugin->buildFinished($build, [@dependents]);
+            1;
+        } or do {
+            print STDERR "error with $plugin->buildFinished: $@\n";
         }
     }
 
@@ -74,9 +78,11 @@ sub stepFinished {
     $logPath = undef if $logPath eq "-";
 
     foreach my $plugin (@plugins) {
-        eval { $plugin->stepFinished($step, $logPath); };
-        if ($@) {
-            print STDERR "$plugin->stepFinished: $@\n";
+        eval {
+            $plugin->stepFinished($step, $logPath);
+            1;
+        } or do {
+            print STDERR "error with $plugin->stepFinished: $@\n";
         }
     }
 }
@@ -115,8 +121,8 @@ while (1) {
             } elsif ($channelName eq "step_finished") {
                 stepFinished(int($payload[0]), int($payload[1]));
             }
-        };
-        if ($@) {
+            1;
+        } or do {
             print STDERR "error processing message '$payload' on channel '$channelName': $@\n";
         }
     }