Slightly different wording for error cases around entropy source selection.

This patch makes the wording around error cases for selecting an entropy
source in Tor slightly more verbose. We also let the user know when
something goes wrong that we are trying out a fallback method instead.

See: https://bugs.torproject.org/25120

src/common/crypto.c

@@ -1903,13 +1903,13 @@ crypto_strongest_rand_syscall(uint8_t *out, size_t out_len)
   if (!provider_set) {
     if (!CryptAcquireContext(&provider, NULL, NULL, PROV_RSA_FULL,
                              CRYPT_VERIFYCONTEXT)) {
-      log_warn(LD_CRYPTO, "Can't get CryptoAPI provider [1]");
+      log_warn(LD_CRYPTO, "Unable to set Windows CryptoAPI provider [1].");
       return -1;
     }
     provider_set = 1;
   }
   if (!CryptGenRandom(provider, out_len, out)) {
-    log_warn(LD_CRYPTO, "Can't get entropy from CryptoAPI.");
+    log_warn(LD_CRYPTO, "Unable get entropy from the Windows CryptoAPI.");
     return -1;
   }
 
@@ -1954,9 +1954,11 @@ crypto_strongest_rand_syscall(uint8_t *out, size_t out_len)
         log_warn(LD_CRYPTO, "Can't get entropy from getrandom()."
                  " You are running a version of Tor built to support"
                  " getrandom(), but the kernel doesn't implement this"
-                 " function--probably because it is too old?");
+                 " function--probably because it is too old?"
+                 " Trying fallback method instead.");
       } else {
         log_warn(LD_CRYPTO, "Can't get entropy from getrandom(): %s.",
+                            " Trying fallback method instead."
                  strerror(errno));
       }
 
@@ -2009,7 +2011,7 @@ crypto_strongest_rand_fallback(uint8_t *out, size_t out_len)
   size_t n;
 
   for (i = 0; filenames[i]; ++i) {
-    log_debug(LD_FS, "Considering %s for entropy", filenames[i]);
+    log_debug(LD_FS, "Considering %s as entropy source", filenames[i]);
     fd = open(sandbox_intern_string(filenames[i]), O_RDONLY, 0);
     if (fd<0) continue;
     log_info(LD_CRYPTO, "Reading entropy from \"%s\"", filenames[i]);
@@ -2019,7 +2021,8 @@ crypto_strongest_rand_fallback(uint8_t *out, size_t out_len)
       /* LCOV_EXCL_START
        * We can't make /dev/foorandom actually fail. */
       log_warn(LD_CRYPTO,
-               "Error reading from entropy source (read only %lu bytes).",
+               "Error reading from entropy source %s (read only %lu bytes).",
+               filenames[i],
                (unsigned long)n);
       return -1;
       /* LCOV_EXCL_STOP */