make missing symbols for coverage tasks more explicit (#554)

This moves from:

```
"Error: coverage extraction from C:\users\bcaswell\projects\bugs\andrew-coverage-fail\setup\oft-setup-5c77cfe1b181520ab0b33a16286a690a\fuzz.exe failed when processing file "11f6ad8ec52a2984abaafd7c3b516503785c2072".  target appears to be missing sancov instrumentation",
```

To even more explicit:
```
Error: Target appears to be missing sancov instrumentation.  This error can happen due to missing coverage symbols.
target_exe: C:\users\bcaswell\projects\bugs\andrew-coverage-fail\setup\oft-setup-5c77cfe1b181520ab0b33a16286a690a\fuzz.exe
input: "11f6ad8ec52a2984abaafd7c3b516503785c2072"
debugger stdout:
...
[+] disabling sympath
[+] processing fuzz.exe
[+] no tables  fuzz.exe
[+] processing C:\WINDOWS\SYSTEM32\kernel.appcore.dll
[+] no tables  C:\WINDOWS\SYSTEM32\kernel.appcore.dll
[+] processing C:\WINDOWS\System32\KERNELBASE.dll
[+] no tables  C:\WINDOWS\System32\KERNELBASE.dll
[+] processing C:\WINDOWS\System32\RPCRT4.dll
[+] no tables  C:\WINDOWS\System32\RPCRT4.dll
[+] processing C:\WINDOWS\System32\msvcrt.dll
[+] no tables  C:\WINDOWS\System32\msvcrt.dll
[+] processing C:\WINDOWS\System32\KERNEL32.DLL
[+] no tables  C:\WINDOWS\System32\KERNEL32.DLL
[+] processing ntdll.dll
[+] no tables  ntdll.dll
Error: unable to find sancov counter symbols [at DumpCounters (line 114 col 9)]
...
```

src/agent/onefuzz-agent/src/tasks/coverage/recorder.rs

@@ -25,6 +25,8 @@ pub struct CoverageRecorder {
     script_dir: OwnedDir,
 }
 
+const SYMBOL_EXTRACT_ERROR: &str = "Target appears to be missing sancov instrumentation. This error can also happen if symbols for the target are not available.";
+
 impl CoverageRecorder {
     pub fn new(config: Arc<Config>) -> Self {
         let script_dir =
@@ -57,28 +59,19 @@ impl CoverageRecorder {
         let script = self.invoke_debugger_script(test_input, &coverage_path)?;
         let output = script.wait_with_output().await?;
 
+        let stdout = String::from_utf8_lossy(&output.stdout);
+        let stderr = String::from_utf8_lossy(&output.stderr);
+
         if !output.status.success() {
             let err = format_err!("coverage recording failed: {}", output.status);
             error!("{}", err);
-            error!(
-                "recording stderr: {}",
-                String::from_utf8_lossy(&output.stderr)
-            );
-            error!(
-                "recording stdout: {}",
-                String::from_utf8_lossy(&output.stdout)
-            );
+            error!("recording stderr: {}", stderr);
+            error!("recording stdout: {}", stdout);
 
             return Err(err);
         } else {
-            debug!(
-                "recording stderr: {}",
-                String::from_utf8_lossy(&output.stderr)
-            );
-            debug!(
-                "recording stdout: {}",
-                String::from_utf8_lossy(&output.stdout)
-            );
+            debug!("recording stderr: {}", stderr);
+            debug!("recording stdout: {}", stdout);
         }
 
         if !has_files(&coverage_path).await? {
@@ -96,9 +89,12 @@ impl CoverageRecorder {
                 .ok_or_else(|| format_err!("unable to identify coverage input filename"))?;
 
             bail!(
-                "coverage extraction from {} failed when processing file {:?}.  target appears to be missing sancov instrumentation",
+                "{}\ntarget_exe: {}\ninput: {:?}\ndebugger stdout: {}\ndebugger stderr: {}",
+                SYMBOL_EXTRACT_ERROR,
                 self.config.target_exe.display(),
-                filename
+                filename,
+                stdout,
+                stderr
             );
         }